If a plumber puts in a boiler but refuses to sign the boiler warranty saying so, can you withhold paying him?

My friend is having trouble getting her plumber to sign the warranty for the new boiler he just put in and everyone is telling her not to pay him until he's signed it. She gave it to him to sign, and he gave it back to her folded up but when she looked at it after he'd gone, he still hadn't signed it, just filled in his name and address at the top of the form. He's a Type A plumber i.e. angry all the time, and she's nervous.