I talk to my tenants about it before they move in. I tell them I understand overnight guests but since we negotiated the tenancy as single occupancy that more than a few nights a week was too much except for maybe an out of town guest a few times a year. I explain the possible choices of renegotiating the lease if they addressed the situation before any actual occupancy like situation took place. I let them know that extra tenants without negotiation are grounds for notice and I don’t have to give a reason when and if I feel it occurs.

Only one tenant abused it, bringing a boyfriend with her on the very first day! It was resolved quickly.

If I were you, I would tell her it bothers you how much her BF stays there compared to the occupancy you negotiated with her. I don’t think you need boat loads of legal precedence and boiler plate behind you…You need to know how she is going to act to your legitimate concerns and either get her to have him over less, screen him and possibly renegotiate the contract, or give notice (offer to accept her giving notice first).

Some may disagree with offering to let the tenant give notice, but it has worked well for me in the two instances I wanted someone out over their behavior.