The problem they had was this (as I understand it, being a landlubber): the sails are raised and lowered on the mast using small rectangular "cars". The cars (pictured below) fit in a slot on the mast, and the end pieces pictured below keep a tight seal between mast and car that keeps the bearings in place. The end caps that my friend had looked like the black one on the left. The cross-piece of the T shape was too narrow, however. When the wind blew hard it allowed a space to open between mast and car, which caused the bearings to spill out onto the deck.
Replacement end pieces could only be ordered from a company in Denmark, at a price of $4 each. With 7 cars, and two caps each, the cost adds up. Worse, delivery was slow, and the catalog didn't show the detailed dimensions, so it was impossible to tell which piece was the one he needed. The alternative was to buy entire replacement cars, also shipped from Denmark, at $90 each.
So my friend came by one evening, and in the course of 3 hours we did 5 design iterations, coming up with a piece that seemed to fit pretty well. I printed two in PLA, and he took them back to his boat to test them out. One more sub-millimeter tweak and we were good to go. I printed out the final set of 14 in ABS, as there was a lot of question about whether PLA would stand up to weathering well.
They're taking the design files with them, in case they need more printed later.