I'm not defending the GP's acquaintance but getting hit by a paintball is lower than the typical risks you accept when you dig around in a dumpster. Generally speaking dumpster diving isn't trespassing unless it's behind a gate or something. It really depends on the dumpster in question.

If it's a good construction or scrap metal dumpster with a lot of nice material in it then defending it with a paintball gun is very much a dick move though.

I don't see why he's getting down-voted. He just said he knew a guy who did a thing. He didn't even imply whether he approved of it or not.