You would probably need to prove that she somehow "forced" him to do it or unduly influenced him. You are looking at a tough fight and will realistically need an attorney skilled in such fights. Your dad could have changed his mind at any point and for any reason, or he might have been misleading you or not wanting to disappoint you. You'd probably need to have a medical professional or two agree that at the time your father changed the beneficiary he was mentally incapable of doing so. This is a hard thing to prove most of the time.
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This isn't an estate matter really - it is a contractual matter of disagreement at this point with the insurance company. The insurance company is going to presume the beneficiary designation is legitimate and follow the binding contract. You will have a difficult time proving he did not appreciate the change - you certainly will need a good contract litigation lawyer to even have a chance.
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This is a very difficult battle to both fignt and win. No way is the insurance company going to do anything without a court order, and that is going to be tough to get. You will get nowhere without an attorney. I doubt you can prevail.