THEFT. Plain and simple. Unless she has giving him the dog, it is not his to give away, or sell or drop off at the pound. Call the police and report it. If he will do something like give away his MIL's dog, what ELSE might he have done or be doing? Get new locks on the doors to her home, garage, etc. Alert the doctors and hospital so that his access to her is limited and supervised, or completely revoked. What is the situation with the daughter (his wife)? Is she lined up with him or with her mother, or caught between them? Depending on her attitude, she may need moral/emotional support from the rest of the family, or you may need to consider her a part of the problem.