Ouch! Sorry to hear about your roommate problems.
Try talking to your roommate again. Point out to her, that if she ends up marrying this guy, she's going to be picking up after him, flushing the toilet for him, etc. for the rest of her life. This guy is taking advantage of her, and by extension, you and your other roommate. Tell her he and his dog are not welcome any more, and that she will either have to tell him not to come over or move out.
If she does move out, you will have to find a third roommate, but that shouldn't be a problem. Rental housing, particularly around colleges, tends to be hard to come by. You and your other roommate may have to cover a month's worth of rent between the 2 of you, but it's worth it.
Good luck!
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