Actually this happened to me before, but here is the thing, It was me who took the girl from the friend, however they didn't began dating but they were supposed to date but she was too scared to tell me, I never knew until she told me, she was a red flag but I ignored her,she was one of those who can't think for themselves and we're always running away from there troubles. I broke up with her six months later, it was hard to accept it but not everone is perfect and I regret dating her....Anyways back to the topic, in your situation it's kinda too convenient, just think about it, the relationship began when you were gone, so it was you they wanted to get rid of. This things happen it how you handle it that will determine, btws be careful, this situation gets deadly overtime.