I had a bisexual friend in university. She was a girl though - so it was less of a problem for me. Eventually, I realized how much her lifestyle was having an affect on me (I used to cry all the time) - and after trying a "let's agree to disagree" phase, where we taboo-ed the topic of sexuality (including my choices of not dating, etc)....eventually..... we turned from being friends to being friendly.
Here's what the problem is:
1. She's a girl
2. She's confiding in you that she's bisexual (you should choose not to have these conversations with her, because I doubt you'll be able to change/influence her)
3. She's expressed a mild interest in religion, so you may think she's your new dawah project ... which is fine, as long as she doesn't become more than a dawah project
4. You're in university - everyone is confused and is "exploring". Most muslim kids don't explore "sexuality".... but most other kids do - you kinda have to leave her alone in that realm, because it can get really confusing and right now .... education is the most important thing.
5. You notice her casually putting her arm around other guys - really... it's not a big deal if she's not Muslim. But if it bothers you even slightly, I would take that as a sign that you should distance yourself from her.
Now your question: If it ever comes up, don't ever shy away from telling her what the Islamic stance is on bisexuality (three-fold: multiple partners = no-no, homosexuality = no-no, pre-marital sexual relations = no-no). Then tell her that since you can't endorse or accept her lifestyle choices, you would rather not discuss these things with her - like EVER. Feel free to share homework - you need homework sharing friends in university