The letter after "500" shows the gauge.
A=12
B=16
C=20
I think the .410 is "D". I know they have one, but I have no experience with one. To the best of my knowledge, Mossberg never made a 500 in 28 gauge. At least not one I've ever heard of.
Make sure he isn't putting the barrel on with the bolt forward. If he does, it won't lock in right. Have it half to all the way back and screw the barrel locking screw in as tight as he can by hand.