I kinda had a similar problem. An old friend of mine went out with my boyfriend a couple of years before I did. It was only a week. But it finished.
Anyway When I got with him, she decided she wanted him. Orginally she never said/did anything in front of me, only to him and my best mate. She usually claimed to be "drunk" but this wasn't always the case.
Final straw came when she was running her hands up my boyfriends leg, in my house. I could have killed her. Lucky my friend distracted her attention, and my boyfriend moved away from her.
Ex's and friends who fancied your boyfriend are never good. I'm not sure what you can do about it. Possibly talk to him about it, see what he thinks.
Best of luck