Have him take a long, rather large diameter hardwood dowel, long enough to reach past the muzzles, and depress the button with it. While holding it down lightly tap on it with a hammer (I'd use a non-marring mallet if it were me, in case it accidentally bumped the muzzles). I'll bet it comes loose.

Probably safer if another person helps by holding the gun with the butt on the floor (on a padded surface) and lightly pulling the forend away from the barrels as he taps on the end of the dowel.

SRH

May God bless America and those who defend her.