Aww man, I really wish I could advise something properly here, could you and her not solve your way around living together in any type of way? even if it was just for a few months, I've lived with my girlfriend for 9 months so from my experience, the thought of her being around some other guy, especially with some guy she would work with I'd not only end up clingy and paranoid, but I'd probably push her away. How long have you been with her? Try and speak to her and hint that you don't want her being room mates with him, If she understood if it was her view with you and a female, then she wouldn't do it.
But the fact of the thread must mean that you're obviously worried about this. Even if you don't see a reason or don't want to be, you still will be, and if she did, and worse came to the worse, then you'd be better off anyway, you'd only need someone who's faithful and trust is key in a relationship. (I'm not in any way saying she would, just that if it ever came to it, then you know where you stood)