I’d definitely get a signed waiver. There’s no telling what is in that wood. I’d also include a contractual obligation on her part that it will never be allowed to come in contact with food, drink, or children. Honestly I don’t use pallet wood because I’ve been in the transportation business for 22 years. I’ve seen where pallets go and what they get into. The same goes for the wood you are working with. I’d be exceptionally leery about that stuff.