Tenant question from inexperienced landlord.

[John H.]

I rented a small house to my daughter and niece at a reduced rate of 800 in Ma. A month later niece’s boyfriend moves in. Within a few months house is full of drama and they aren’t getting along. I move daughter out into another house. Fast forward a year, I had to pay almost $1k to keep electric on during the winter as they fell behind. Niece split up and moved out leaving boyfriend alone in the house. He just asked for a therapy dog and I said sorry, still no pets. I went to talk to him yesterday and state that he will need to pay market rate for rent as I can’t subsidize his rent as my kid has long moved out. Before I could mention the rent increase he states he has cancer and will be starting treatment. He is a great guy and my heart goes out to him. The hard part is at $800 I am losing money. Now I’m questioning if I’m even able to say no to a therapy dog. It’s a fully rehabbed house with nice wood floors. Selfishly I’d hate to lose money while he’s there and then have to deal with thousands of dollars of potential dog damage in the end.
Any thoughts this, and regarding a therapy dog, am I even allowed to say no?