Martin - there are 4 and 5 stud trailer hubs. Most of the 4 stud are 5.5 INCH PCD which is ummm.. *counts* 140mm (as long as I've woken up enough to work out that math right...)
Any wheel will fit if it has the correct PCD, so he could put 13s on if he can find them. How big is the boat (weight wise) because trailer tyres have load ratings like car ones, so if he can work out what tyre he needs, he'll know what rims he needs. My suggestion would be getting tyres/rims that can handle at least half that weight again to take anything else on board into account (fuel, rods, air tanks, blah blah blah, you get the point).
That's really all I know about boat trailers, and it's been far too long since I've really has much to do with them, but hopefully that's SOME help.
The 5 studs I can't remember the PCD for though, so you'll have to hope he has 4 stud
Failing just living on hope you could get your Google on and show up that MUZT3K fuckwit