Even having simultaneous crushes on the same guy in our early twenties couldn't sour our relationship. We began to think that we were indestructible.

That was until Melissa and I started working for the same company. I had quit an awful job for a large retail chain and looking for something completely different, started working as a personal trainer for an up and coming 'trainer agency' of sorts.

I was well versed in the art of fitness but my new employer paid for me to get my Trainer's Certificate and I was over the moon with my new life.

Melissa and I had become obsessed with working out ever since graduating high school. We had battled with countless ineffective diets and exercise regimes through our school years and had vowed to work out together and motivate each other forever more.

Melissa confided in me that she needed more money as she was looking to start a family in the years to come with her new husband - Phil - and that's when I suggested that she come and work with me.

Melissa was in excellent shape and she was so enthused about the new job that she took herself off and completed the Personal Trainer course on her own time. My employer was so impressed that she took Melissa on with very little prompting from me.

Being the friend that I was, I generously gave Melissa half my client base (gradually so that Melissa wasn't overwhelmed) and she took to her new role like a duck to water.

I had lost work and thus money by cutting my client list in two but I felt good about helping my friend and I slowly but surely, with a great deal of hard work, rebuilt my client list over the course of about a year.

Melissa started to change about a year into the new job. She started getting a bit preachy about diet and fitness and had rebuked me on more than one occasion for eating too many carbs.

“What will our clients think if they see you stuffing your face?” she had said sort of jokingly at first but with gathering intensity the next couple of times.

I took these outbursts in my stride but I was taken aback to say the least.

Then Melissa started with the critique of my physique. “Your obliques aren't as tight as they once were Jen, do you think you're getting a bit lazy?” was one of the key pearls of wisdom that really shocked me.

She also basically yelled out one morning whilst cycling behind me on a local bike-track, 'Wow - your butt's hanging over the bike-seat Jen, more biking and less bickies should fix that'. I saw red.

The final straw was when I went on holidays for two weeks to Fiji with my new boyfriend. Melissa had very eagerly offered to 'take care' of my clients while I was away.

I reluctantly agreed as I felt as though our friendship had cooled somewhat but a little voice in the back of my mind convinced me that trusting Melissa might be the first step to repairing our fractured friendship.

I also thought that some time away might help us to take stock and surely we would be better friends on my return. I have never been so wrong in my life.

When I returned, I found that Melissa had not only taken care of my clients, she had basically taken them over. Upon contacting some of my ex-customers, it seemed that Melissa had told them that I had called her and told her that I wasn't sure that I was ever coming back.

She had pitched herself as the harder working trainer (having never had any time off since starting the job) and she had also started under-cutting my rates and throwing in some extra perks like free training sessions.

I told my now ex-clients the truth but only the faithful few returned to me. The rest were either lured in by Melissa's fringe benefits package or so confused that they actually took their business elsewhere.

I confronted Melissa but she had become extraordinarily cold all of a sudden and she gave me the 'you snooze you lose' line.

I confided in my employer about the situation but she had said that in this business it was 'the quick or the dead'. I was getting so sick of clichés!

I decided that if this was the way it was then all I could do was play along.

I started secreting away quantities of cash from the cash-box in the company safe at the office. All trainers had access to the safe so that they could put the company share of payments away until the end of the week when our boss would finally get around to tallying-up. My employer was a self-made business woman and the operation was pretty low-tech.

I started withholding small amounts of cash at first and nothing was said. But soon I was keeping larger quantities aside and that set the cat amongst the pigeons.

Everyone was a suspect. Staff meetings were held about the missing money and no one was directly accused but I was privy to occasional water-cooler gossip and it seemed that all eyes were on Melissa after what she had done to me. It was perfect.

I then felt that it was time to swoop. I went to see my employer in private and basically told her that Melissa had a problem.

I said that she had been a bit of a kleptomaniac during high school but I thought she had since 'grown out of it'.

My employer wasn't stupid and she asked me if I had a vendetta against Melissa but I delivered an academy award winning performance. I said that initially wanted revenge but now that I could see that the stress was getting to Melissa, causing her to relapse into old behaviours, I just wanted to help my friend.

I also said that if Melissa was going off the rails that it would reflect poorly on the business. I mean, how long would it be before she started stealing from clients?

Then it came to me. I sort of suggested some sort of intervention. I said that we should all try and help Melissa, knowing that my employer simply wanted to find out who was causing all this chaos and that she would agree on that basis.

I brought the stolen money into work, wrapped in an elastic band and stuffed into an empty ibuprofen box.

I 'bumped into' Melissa in the locker room and I pretended that I wanted to organise a girl's night out to sort of re-ignite our friendship which had become almost non-existent.

Melissa seemed genuinely eager to do so and I felt a sudden pang of conscience. But then she headed for the showers leaving her sports bag unattended.

I fiddled about with my stuff long enough for Melissa and I to leave the locker room together. We headed for the exit and everyone was gathered there with our employer at the centre of the melee. I had told her that a random bag check would prove my theory right.

Everyone emptied their bags at the same time under the watchful eye of our employer. When the ibuprofen box tumbled out Melissa's bag I quickly glanced in my employer's direction.

She picked up the box and found the cash inside to the tune of Melissa's protests (which only made her sound more guilty). There was over fifteen hundred dollars in the box.

Melissa was fired on the spot and my employer only agreed not to take legal action because I begged her not to; feeling as though the whole thing had gone far enough.

Melissa called me and accused me of setting her up. I acted as though she had gone mad. It goes without saying that our friendship ended there and then.

I have since become a freelance trainer and I have never told anyone about my revenge on Melissa. I sort of feel guilty sometimes but part of me also feels that Melissa had it coming.

All names have been changed. Picture posed by models.

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