The only insurance likely to pay out is the builders IF he has any and that will probably need a claim against the builders.
True, it also assumes the owner has receipts for the work.
I would doubt that would be likely to succeed as the builder suggested the valve was removed and the owner agreed!
I doubt that they had mentioned that a probable side effect was future sudden flooding with sewage. The owner could not be expected to have known this; the builders should have known what the valve was there for.
It would be similar to a repairer removing flue components from a boiler, spilling combustion products into a house; if they don't know what they're doing, they shouldn't be working on that equipment.
