They probably just want to know whether YOUR WIFE is planning on making a claim against your father.
Your health insurance policy undoubtedly has a provision that gives it "subrogation" rights against anyone who causes an injury to you or your wife resulting in the insurance company having to pay medical bills. If your wife makes a claim against your father alleging he is liable for her injury, your health insurance company has the right to "piggy back" on her case, so if & when she gets a settlement, the health insurance company would get repaid the medical expenses it paid for her, right off the top.
So when an insurance company pays medical bills for an injury related to a car crash, a fall down, or other circumstances that suggest there MIGHT be liability in some third party, they typically send a questionnaire form to their insured asking a bunch of questions that allow them to figure out whether there might be a liable party out there who is ultimatley responsible for the injury, and against whom they could bring a claim to seek reimbursement. That's probably what they are sending you. And although the insurance company theoretically could bring its own claim against your father directly, IF the facts of the accident indicate he was negligent in some way, they usually do NOT do this. Rather, they wait to see if the injured party is bringing a claim, and if she does, then they assert their right to be paid back out of any settlement money.
Short answer: Go ahead and answer the questionnaire form. In this day and age, you don't want to give the health insurance company any more reasons to cancel your coverage--they already have enough. And the chances that you will in any way detrimentally affect your father seem extremely slim, as long as you tell your insurance company that there was no one "responsible" for your wife's accident--she just fell--and that you do not intend to pursue any claim against "the property owner."
Feel free to call if you need any additional advice.