I am an engineer, and to be honest, my job is half getting things to work and the other half is covering my ars@. We know that something will work fine, but because your signature is on something, you over-design it for that 1% chance that things may go south. The worst thing that can happen to an engineer is getting a phone call 20 years in the future and being told to attend a hearing because you signed off on something. You can go to jail and lose your ability to practice engineering anywhere.
When an accident occurs, people and especially corporations, NEED someone to take the blame. Convenient scape goats are guess who? The engineer. So to cover yourself, you add things that would never have been needed in the first place, so that some board in the future can't find a way to throw you under the bus. This is why an engineer on the mine gets close to 3M a year with some 5 years of work experience, because his signature is on everything. The mines, for example, are more than willing to pay him that because when things go south, they have a fall guy.
I was a lead designer for winders at certain mines that carry some 100 mine workers down a shaft, some 1 to 2km underground, and the thing runs 24/7. All the schematics have my name and signature on them. If an accident occurs, I will be called in, even though I am working for a different company doing completely different things.
Will his roof hold the panels? Probably yes. But his signature is on the certificate so he will make the guy pay the 20k to safely cover himself.
I agree with everyone on here, though, he must pay and get the roofing support done. Should have bought a free-standing home if he didn't want to deal with stuff like this.