I'd verify that your neighbor was the original purchaser and didn't do any mixing and matching. Read the link below. Properly stored GP should still be good to go. If there's any doubt or uncertainty, dispose of it or decline your neighbor's offer and let him deal with it.

If your son is just starting out on reloading, he should do some due diligence and seek out someone who *knows* what they are doing. Once savvy, it is easy, safe and rewarding to roll your own....

The best way to dispose of deteriorated smokeless powder is to burn it out in the open at an isolated location in small shallow piles (not over 1" deep). The quantity burned in any one pile should never exceed one pound. Use an ignition train of slow burning combustible material so that the person may retreat to a safe distance before powder is ignited. --

http://www.alliantpowder.com/getting_started/safety/storage_handling.aspx