I would ask him when he last sprayed the wood he gave you and with what did he spray. Once you know what he sprayed, you can find out what it is. Some of that stuff is very safe and I'd have no problem using it. Some is oil based and if recently sprayed, could be a problem. Some of it can be sprayed right on the fruit and is safe for human consumption if it sits for a short period of time.
Personally, I would not worry about it. I get wood from a local orchard. I'm certain they spray but I just don't see it as a big deal. That's me. I guess I figure if it's safe for a food producing operation, it's okay to burn. You'll need to do some investigation to come to your own conclusion.