I say a person can do anything they want when they want to badly enough. I can tell you right now I wouldn't think twice about it if I lost my left thumb, and I'm left handed. I love paddling too much. Heck, I'd figure a way to paddle without my left arm if it came to that.

And now, after having spouted off on something I know nothing about -- thank heaven, and condolences + most fervent hopes and good wishes to your friend -- I'll also say that I use nothing but GP's in sea yaks, and very often as though I have no thumbs. I somehow developed the habit of "palming" my paddle and leaving my thumbs out of the deal altogether as a way of training my arms and hands to loosen up and put a stop to the vise-grip thing. It's a lot of fun, actually, and delightfully effective.