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".