Your a very smart man.
she has a boyfriend and she is telling you she is going to be lonley if you leave.
well it does sound like she has some feelings for you. treat her friendly and kindly
and if and when she leaves her boyfriend then you can make a move
but dont get to into her. because oneday she could just stop and try and make htings work out with her boyfriend
its a hard game to play
Good luck
Regards