I used to run the gelcoat spray booth at a major production facility, and having done this before I can tell you there is no fix other than a 5 gallon bucket of acetone and a bundle of rags. Be very thorough and make sure all of the uncured gelcoat is removed, it will never cure and any gelcoat you put over it that does cure will almost certainly alligate. I've seen people go to great lengths to try to solve this, inlcuding spraying straight catalyst on the uncured gelcoat within an hour after it was sprayed. Nothing works, you're not gonna get catalyst down to the gelcoat that is actually touching the substrate, which is the part that matters. Worst case scenario, you overcoat it with catalyzed gelcoat, which cures nicely with no alligation. The uncured stuff will still be trapped under it, and eventually it will all let go, clogging your limber holes with gelcoat chips. Nope, there's no easy way out of that one. When you bust out the fiver of acetone, be careful of ignition sources, that much acetone fume is dangerous stuff! I'm still laughing at the guy in the yard who tried to provide ventilation for acetone fumes during a wipedown of his bilge
with a shopvac. The shopvac was sitting under his open forehatch, and when it exploded the lid shot out of the hatch
about fifty feet in the air! Even fans will provide ignition, be careful.
Oh, and make sure you use surface seal in the last coat for proper curing. If your buddy didn't know to catalyze gel, I'm guessing he's not familiar with waxing it either.