This is the case it was based on, from Wikipedia:
Thomas Montgomery, a 47-year-old married man was convicted in 2007 of murdering a workmate in a case called the "Internet Chatroom Murder". He posed as "Tommy", a 21-year-old marine, and began an online relationship with a 17-year-old called Jessi. A workmate of his, 22-year-old Brian Barrett, subsequently began an online relationship with the same girl after Montgomery's deception was revealed, and Montgomery shot him in their work
car park. "Jessi" was actually a middle-aged woman who had been using pictures of her daughter.