When you introduce someone to one of your friends, and they steal them from you.
Stealing them from you is defined by the stolen friend spending less time with you than usual, and more time with the friend doing the stealing.
Ste hangs out with Bryony. Ste introduces Xanthe to Bryony. Bryony now hangs out with Ste less in favor of hanging out with Xanthe. Xanthe is therefore a "Friend Thief".
Introducing one friend to another, only find out that the two friends enjoy eachothers company more so than that of the original Introducer. Thus leaving the original friend of both, friendless.
John: hey Jake what are you up too?
Jake: oh just hanging out at my house, playing video games with Wally.
John: Why didn't you call man? You know I wanted to hang out tonight.
Jake: Well Wally was already here when I got home.
John: Wally is such a friend thief.
when you introduce 2 of your good friends to each other, and they both leave you behind and spend more time with each other
"Jim introduced chris to sophie, and they got on well. so well in fact that they left jim behind! Chris and sophie are both friend thiefs"