Thanks for the replies.
The problem is, there is no way I can see her again and ask her number, as she is at another campus (no chance of meeting her during classes etc), so I have no direct contact with her. The only possibility would be if there were a group meeting again, where this guy invites her to. Which is quite improbable (I barely know him, so I can't really arrange going out with his group of friends)...
EDIT: Braisler, you beat me to further clarification ;-)