Father to house sit

[Adam Rockstad]

I have a young couple that rents from me, and they are going to be leaving town for three weeks over the holidays. They are going to leave their dog behind, and want his dad to stay at the house and watch the dog, and their house. I am fine with this, but I was wondering what should I do to protect myself while he is staying there. His son and his girlfriend have a 12 month lease with me, so I do not think that I need to have him sign a lease, maybe some type of addendum to the lease, or what? Anyone have any input on how I should handle this. Thanks in advance.