Should I be worried??

So a couple a years ago my boyfriends slept with this girl (this was way before I knew him) and when they slept together she was cheating on her boyfriend with him. She's in his group of friends and was then and they've been friends after but stopped for a while until recently when he turned 18 he obviously went out with that group of friends including her. Since then they've had a couple of conversations on Facebook and I've seen the messages it's nothing bad but I don't know if I should be worried. If I say something will I be fighting a losing battle?? I need advice:(