You probably won't get too much of a reply to this here, most of the boats here aren't bottom-painted. I think the best way to remove bottom paint is to pay somebody else to do it.

However - if he's a do it yourselfer, and he has that many layers of paint, I would recommend the paint stripper option. There is a type that you slather on the stripper, then cover with a paper backing. After you let it sit for awhile you peel away the backing and toss the mess.

Bottom paint is nasty stuff - make sure if he sands or power washes it off he has a way to collect the debris, and adequately protects himself.

Funny story - back in the days when we had a keel boat I had the yearly ritual of sanding the old paint and putting on a fresh coat or two. I got in the habit of buying disposable Tyvek painters suits that cover you from head to toe, and also using a respirator when sanding. I usually wore the suits when painting as well. I generally refer to them as "bunny suits", 'cause you only need to add some ears and a tail to become Harvey. One of the last times I did this, I was finished for the day, peeled the suit off and head to the bathroom. In there is a guy completely covered from head to toe in blue bottom paint dust and paint. We talk about how messy bottom work is, and he notes that I barely have a speck of paint on me. I reply that was because I wear bunny suits when I do the work, and then I recommended that he try it. He says "hmm, just go to the Hallmark store and get a bunny costume? Sounds like it oughta work...". I had to work hard to control myself as the mental pictures of this guy in a bunny costume doing bottom work just about made me lose it laughing. I paused for moment considering whether I should agree so I could get to see that, or be nice. In the end I was nice, but whenever I work on the bottom of a boat I think of this...