Unauthorized Additional Tenant

[Mark Smith]

I'm going to try to make this as short as possible, but I'm looking for some advice on how to handle a situation.  I have a tenant in one of my units who lives there with her two daughters.  She is separated from her husband, who did not live with her and is not on the lease.  He seemed to be coming around the property more, and seems to be there every time I'm there.  Today when I stopped by to drop off some things I noticed an additional vehicle in the garage, which I imagine is his since the woman and her daughters are the only tenants in my two family at this point.  In the past he has denied living there, and the woman has denied that he lives there.  I took over the tenant when I bought the property, and the lease calls for a 25% increase in rent if there is an additional tenant.  I don't necessarily agree with this clause, but it's in the lease and she signed the lease.  I'm not sure how I go about proving he lives there, and as this is my first property (and first tenants) I'm afraid of some sort of retaliation if I "stir the pot" so to speak.