That is not the point. For the fourth time, should she have paid this plumber when he failed to find the leak. That is what she called him out for.
More relevant, why did the push-fit fitting leak, when was it installed and who installed it? No inserts, wrong inserts, cut with a hacksaw, pipe too short, mice? If it's recent and incorrectly fitted, she should be chasing the installer for the repair costs.
Some of the ceiling would have been knackered by the leak anyway. There are also a bunch of further suspect push-fits concealed in the building structure. If she gets the installation pressure tested to 10 bar (as should have been done when it was installed, but probably wasn't) there may be more repairs to be done if more of them fail.
PS And yes, the plumber didn't have a crystal ball and didn't know where to dig. If he spent 3 hours looking for a leak, he'd charge for 3 hours labour. The leak can be in one place and the water drip off the pipe yards away. How else can you look for it without ripping down ceilings or ripping up floors?
He should have told her that (maybe he did) before he got the crowbar out.