He has a few options.
He can have it soda blasted, which may or may not effect the gel coat, depending on the blaster. He then get dirty sanding, then sanding some more, then sanding some more and then polishing polishing
He can dirty and use a 4" push scraper to get it off, then sand, sand, sand, and polish
He can paint it with white bottom paint and forget about it.
He can use chemical strippers. These are very much temperature (read weather) dependent and they only seem to work in the weather in which you want to be boating. There are many on the market, and I have tried just about all of them with very mixed results.
No matter how you look at it, it is a labor intensive job and getting the paint off is only part of the battle.