How to deal with flirtatious "friend"?

There is a girl who HAS her own boyfriend, but always flirts with my friend's boyfriend. She doesn't outright hit on him, but she always has that seductive countenance and giggles and acts so pitiful and asks for help with homework and stuff. My friend's bf is complete oblivious to this, and is just nice, but the behavior of this flirtatious girl is very very annoying to my friend, and I feel very bad for her. She has asked me for advice on what to do about it, and I honestly have no clue! Can you help me?