What to do about a potential problem tenant.

[Dolores Gonzalez]

My tenant is a "single mother" with small kids. She has only been there for three months. Her kids father is seen at the apartment often, but she assured me that he doesn't live there and only comes over to help with the kids. I don't believe it. Now he is causing disputes with the other tenants. A neighbor has said that the police have been at the building several times. I've had a conversation with the tenant about her kids father and a few other issues like keeping the common areas tidy and disposing of trash properly. I feel like I need to start documenting everything and send a letter. Any suggestions on how to handle and approach?