Looking to help diagnose for a buddy, he's used a Evinrude 28 jet from 1995 for a couple years without issues but last couple months when he's taken it out the motor will run fine for 5-10 minutes then randomly die and wont turn back on. If it sits for a few days, it'll start back up fine and will do the same thing (die after 5-10 minutes of running fine, then wont start back up)

He's taken it to his mechanic and he couldnt find anything wrong with it, any thoughts to what it could be?

Might be the Power pack getting hot and acting up. Replace is only option if so. Probably 100 bucks or so.
Is the water pump good? Could be overheating too.

check fuel pump for cracks in the diaphram.

change any filters.

double check all fuel connections for loose, any air at all will cause the fuel system to screw up.

what fuel line does he have on it? check for delamination on the inside, ethanol will do that to ya.