That's a sad story.... and it would be even sadder if you retired from modding. Whoever sends you his watch needs to understand that you are not a professional, with 10 years of training on your back nor are the watches you're working on genuine articles with proper factory QC's. To have the expectation that this rep hobby is like buying a rolex at an AD and taking it for service to an AD is childish. You can only try to give it your best and if in any particular case it turns out bad, be ready to make up for it. Nothing more, nothing less.
The other modder you referred to very recently explained his relationship to one of his main customers (and friends) and basically said that one of the cornerstones of their "working" relationship was the common understanding that if you comission mods, you have to accept that there's are potential risks involved. I totally agree.
You where prepared to make it right, redo the mod, even refund the money. If your customer expects anything more, he's clearly on the wrong path...