What can I say to my friend who's in this situation?

Sit down with her over a hot chocolate and marshmallows and just start with some friendly chit chat. Make her trust you. Then just explain the situation from your point of view. tell her that he has a girlfriend and he dumped her for a reason. Tell her that if she stays this obsessed with him she will never get over him and wont be able to move on to another guy. tell her she is a friend with beneifts to hima dn that is all. He is taking advantage of her big time. Tell her that it is for her own good. hope this helps. :)