If his gun truly is in As-New condition, with only 25 shells fired from it.....
Realistically, he can probably sell it for $700-850...... Maybe not on MWT, but on an auction site like GB.
If it's in a more used condition price will drop considerably. It's funny, but having the box will always help sell a gun like that on an auction site.
Problem with 1100's like that, they are limited to 2 3/4" shells, which for most people makes it an upland gun. 1100's are fairly heavy for upland guns, which makes them less desireable. Sub guage 1100's are the ones that pull a premium, if it was a 410, 28, 20 or 16 gauge, you can double or even triple what you will get for a 12gauge.