I’m not a roofer but a consumer myself so my opinion will differ from “roofers” who do this for a living. If this was my roof I would be pissed and I would not feel it to be my obligation to pay for repairs. This guy does it for a living, he should know what will happen and advise customer in advance of possible outcome.
If the boards were that weak he should not have agreed to do the job w/out stating they need to be replaced or he would not be responsible for damage to existing planks.
The customer should have been told this may happen and proceed from there before work was started. Doing the work and causing damage like this then saying it is normal while the customer is in the dark the whole time is ignorant on the installer.
I say make him repair for free and next time he wants to undercut a professional he could explain “why” his install is so cheap to begin with.