Protect myself from my Father's Girlfreind

[SV303]

My father has an on again/off again girlfreind. She lives with him, but he kicks her out often due to her alcoholism. My father has recently became ill. My house is in his name because he is carrying the loan. He has a will that in the event of his death, he leaves everything to me. I am worried if he passes, his scandalous girlfreind will try and take everything claiming common law marriage. We live in Colorado which I believe recognizes common law marriage. What kind of legal forms can my father comeplete that will prevent her from getting any of his Estate and comply with my father's wishes of giving everything to me? Since he is alive right now, I thought we should take care of this immediately. Please help