Difficult situation in texas

Allow me to apologize for the length of this post before going any further, this is a tough time for my family and I realize that it is a complicated situation and all of the advice received is greatly appreciated.

My mother is terminally ill and it has become a question of when not if she will pass. Around a year and a half ago she inherited a rather large sum which is in an account at a large bank and safe at the moment. She has been living with a man whom she is just friends with in texas and he has decided that he is now common law married to my mother and wants to make all medical and financial decisions for her. He has no job, and was homeless when he met her, and has been living off of her ever since. He is the only person with access to her apartment and her banking information as well. I managed to have her account frozen seeing as how its in her name and she's not able to oversee its activity.

She has no will, and is now not capable of signing power of attorney over to anyone because of her mental state. To make matters worse she's still married to my father even though they have been seperated for almost 25 years. She has told me over the phone repeatedly that the money was mine if anything were to ever happen to her, she has told my grandmother that she wanted me to have it as well. Im affraid that even if she could draft a simple will that it would be thrown out due to her mental state.

I guess the questions are:

If she's still married to my father is this stranger just wasting his time attempting to become an heir?

If my dad doesnt want anything to do with her estate what do I need him to do to deny any benefit?