roomie bringing home strangers

My take on it: Talk to the landlord to see if he/she would be amenable to letting you out of your share of the lease, given the situation. If Best Friend asks why, be honest that you don't feel safe with her bringing guys home from bars. Above all, don't let yourself get cornered by one of these guys no matter what you have to do to get out of it.

As a mediating step, you could sit down and talk to her about how bringing these guys home makes you feel unsafe and see if there is any agreement you two could come to that would allow you both to keep living together but ensures that you feel safe in your home. I'm sure that if she really is your friend she'd want you to be honest about something like this and that she'd be willing to work with you to make sure you feel safe.