Today I hung out with three people -- one of those being a girlfriend of one of the guys. After they all leave my house, the girl texts me:
"Thanks for inviting me over to your house. Sorry I was sick today, and a little disappointed."
"It's just that [guy] suggested we watch a movie after [school event] on Tuesday and I guess he forgot."
"Its not that I don't enjoy hanging with you guys, it's just..it's V-Day. lol I'm sorry for hounding you with my problems.
I feel I can talk to you openly 
"
This is the first time she's ever told me she's been having relationship problems with my friend (but I've heard about it through other friends), so it's not like I'm her emotional tampon. She's been giving me hints, and saying things like "why am I dating him".
Point is, I'm not going to let her mix-signal me and start telling me about her problems while my friend sits around and has her for the sake of having. At the same time, taking your friend's girlfriend is a shitty thing to do, isn't it? What should I do?