He has to experiment with the DIY or throw the fins away.A large washer will spread the stress over a larger area at some possible expense to foot area, this is something I would do if I were in a far off land diving just to get through but not at home except in the pool. I’m at the opposite end of the shoe size than Simon so I tend to have plenty of spares here.
I would try to save it first. Because I only have one pair of fins.