But could the customer legitimately say that it's the moisture caused by the WFP that's caused the problem?
This supposed moisture has just appeared, therefore, get a double glazing company to inspect the door immediately, he will be able to tell you if this break down is recent or has been down to prolonged wear and tear.
I would also ask the customer who fitted his new windows and then call them and ask why the customer didn't get the patio doors changed, was it because of budget??
I think this is someone trying it on, I would type a letter out explaining the complaints procedure, explain to him that all relevant actions will be taken to investigate this claim and should the claim be found "Not Proven" you will seek compensation from your customer for expenses incurred. Take this letter to him and get a signature with date saying he/her agrees to this.
Then if he accepts and the claim isn't your responsibility take him to court and if it is proven your fault get your insurance comapny to deal with it.
Trev