The zipper is purchased from another manufacturer, so that is not necessarily the suit manufacturers fault, although they do get to choose who the manufacturer is. With two failures I would also start questioning the person using the suit about proper maintenance, usage, and storage. All zippers need to be lubed often.
How come the suit manufacturer isn't just repairing the drysuit by replacing the zipper?
I am surprised no one mentioned silicon. Super comfortable, doesn't leak, and can be replaced in the field in two minutes. Only downfall is it is more fragile. I have never gotten a hang mans neck from silicon, but sometimes did from latex. Neoprene leaks too much for me.
Today for surface suits Gortex is where it is at too. I got a DUI one for free that needed a zipper and seals. Being able to sweat and not have to live in it all day is good for comfort in so many ways.