I'm sorry to hear that.
As a roommate, sharing a residence, each of you on the lease has an equal right to the property, and because the lease is not between the three of you---but with the landlord---you as co-tenants have no way to legally enforce the rules of the landlord as against a co-tenant. Only the landlord can do that.
As the woman is a guest of a co-tenant, there is nothing that either you or your fiancee can do to prevent her from coming and going at her leisure. Your sole remedy is to sever the co-tenancy relationship as soon as possible.
It may be that the landlord will allow you to move to another unit/residence, but if that is not possible, then all you can do is wait until the lease is up and not renew it with this roommate.
Presuming that there was an agreement between the 3 of you that you would peacefully reside in the home together, if the roommate's violations of the rules causes the landlord to evict all of you, you might consider filing suit against the roommate for damages
associated with the need for you to relocate, and under the theory that the roommate, as a co-tenant, has breached the contractual agreement that you had, resulting in monetary damages to you. I'm afraid that absent a good, solid, roommate contract providing other remedies to you, that you really don;t have any other viable options.
Here is an example of a roommate agreement that you might consider using in the future should you ever find yourself in a need to share a residence with a non-family member:
I wish you the best in 2011.Because I help people here, like you, for a living---this is not a hobby for me, and I sincerely ***** ***** abiding by the honor system as regards ***** ***** I wish you and your family the best in your respective futures.
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