Dig a whole and fill with compost if you want to be safe.
It is hard to know how much manure he adds to a specified area, weather the manure is well rotted or fresh and from which animals it came from eg Cows, pigs or chickens etc or if it is a mix.
Another concern is does the farmer spray with pesticides or herbicides, if yes then you don't want to smoke that stuff and secondly whilst spraying you weed may get spotted.