My father named me as the executor on his will and to receive the family home when he dies. My mother passesd away several years ago. He has a girlfriend living with him in the family home that has a lot of influence on him. Would it be legal if she get him to change his will.

Yes. Anyone can change a will at any time.

Depending on where you live, there may be legal requirements to leave a certain proportion of the estate to spouse or children. After that, it's all up to him. If he wants to leave most of it to his girlfriend, or to the local cats' home, he can.