I understand your dilemma.
I think I have to say, that tcmtech and I would be of a similar mind.
I had an ESAB welder which was faulty and during reassembly, there was a short between the control circuit ground and the power mains earth. This took out a few diodes and was hard to work on so I hoped the customer would just go away. It took about 18 months for someone to start pressing me for a resolution. I found a couple of simple IC's were US and replaced them but while finally checking things, a puff of smoke came from the SG3525? control IC. I wasn't able to get a surface mount device so I opted for a DIP package and had to mount it end-on and glue it to the board. Connections were done between the IC and the PCB with 0.2 mm wire strand. I felt like I was working in swiss watch factory. Nevertheless, all was well in the end, and I tried out the welder with some 4 mm welding rods and it worked like a dream.
However, I have an 'italian job' which is the most idiotically complex double sided circuit board you could imagine; it is almost impossible to trace the circuit tracks. It has been in my shed for about 7 years and is going nowhere fast. A bolt came loose in this welder and blew the crap out of everything. I got as far as replacing all the big mosfets, but at the operating test stage, smoke comes from the guts of it and it is too dark to see what is what. I think the current control is at fault.
So, if you want to press on, you will certainly need the proper circuit diagram. You will also need to have an idea of the price/availability of the major semiconductors.
You will also need to consider that you may have an unforeseen disaster with the repairing.
So I think you need to consider the story to tell your customer. This would be something like;
" the design of this thing is rather complex as it is a high speed, high power, electronic circuit, and for faults in the high power section, these designs are almost impossible to repair, because of the possibility that some components MAY have been damaged as a result of the original fault. It may be possible to repair, but it is quite likely that within the warranty period, that another failure would occur and such a result would damage both of our reputations. If it is your wish to proceed with the repair, I cannot promise I will be successful, but my preferred approach is to advise you not to proceed any further. It is the case with modern manufacturing, that products are generally cheap to make and assemble, but they can often be uneconomical to repair "
I thank you for your confidence in me and sorry it is not of much help. Post 2 is worth looking again!