Things to check
The O ring , did it rip ?
Is one of the ports in the barrel pluged ? clean with a tooth pic
Is the gas compinsator on the barrel ring properly ? snap it back in place
Are the 2 steel barrel seals on in the corect order ?
http://www.remington.com/support/parts/model_11-87/important_notice.asp
Has he cleaned the carbon off the mag tube ? spray some oil on the carbon , wipe it off with a Scotch Brite pad and wipe the mag tube DRY
Remove the spring and follower from the mag tube and LIGHTLY , and I mean LIGHTLY lube it and check for crud before reasembeling
John