What to do about person making someone do a will for their gain

My dad recently passed. He had a friend help him occasionally. She took advantage of his poor health in the final month of his life to take him & make him do a will making her sole beneficiary. She also kept me from talking to him the last moth he was alive. I only found out about it when I went by to get his personal papers after his death/ She would not produce any just said she has a will saying he gave everything to her. I have never had a bad relationship with my dad, I'm the only family he had close by.