Unless the 3.1 is a 'retro' in which case he would not need our advice he isn't going to have a USB. Even a CD if present is most unlikely to be a boot device. So taking the HDD out would be required. Easy enough on most mopdels even without exactly the right tools unless you are completely ham fisted. Getting it back together however... While my advice to retainXP and now Windows 7 if they are good enough for your needs and you avoid the risks was perfectly serious not just to wind up Mixture, Windows 3.1 is a bit more of a head scratcher.
The non destructive alternative as said is floppy's - if his drive still works and if he has somewhere else to create them which may be the biggest show stopper. One of the several universal MS-DOS disks (eg Bart's) would probably do the trick just to get files off. Maybe a PPruner with time to spare would create and mail you one - or even a selection that could get your files off .