Something is definitely fishy here. Why is the customer's father-in-law involved at all? Did the father-in-law pay you for the flooring? Even if he did, it's still a personal expense, not a business expense, or it's a gift from him to his son-in-law or daughter-in-law. Either way he does not need your EIN.
You never came across a homeowner asking for your EIN because there is no reason they would need it. They are not going to send you a 1099-NEC for work on their personal home.
At risk of making the customer angry, you could ask them to have the father-in-law's accountant call you and explain what he is planning to do with your EIN. But if you don't want to risk losing the customer, you could just give them your EIN and let them send you a 1099-NEC. As we have said before, it won't harm you in any way. It sounds like the father-in-law is planning to deduct the cost of the flooring as a business expense on his tax return, but you are not responsible for what he does on his tax return.
