Is it your apartment or is it an apartment that both you and your roommate have signed for? The reason I'm asking is because you might be able to get away with more if it is explicitly your apartment, however I am not a lawyer so keep that in mind for the duration of my post.

Would it be possible to create a document that outlined who is at fault [your roommate] if bedbugs are brought into your apartment? The next step would be sitting down in front of your landlord and having all parties sign it and have it filed away. If I had to guess I would assume the agreement doesn't afford any actual legal protections, the idea is that it is just a formality that brings the issue to the landlord's attention. Landlord's hate bed bugs, it is the bane of their existence and can create huge issues depending on how connected and easily contaminated juxtaposed and neighboring apartments or units are. There is a chance the landlord might step in and force a solution that provides your apartment with better protection.

I would also look into any literature that outlines a guest policy and who is entitled to invite guests to stay, their allowed duration of stay, and any rules regarding that. There is a chance that it could simply be as easy as you saying she is not welcome in your apartment and that is the end of discussion. If your roommate violates that and brings her in it could potentially escalate to an issue with the landlord or police depending which route you want to go.

If they engage in any regular behavior that would potentially violate your lease such as smoking pot inside or anything like that I would potentially catalog that as well, not because it is illegal, but because if you need to break the lease or force him to be broken from the lease it could provide an easier avenue of doing that.