Personally, I wouldn't be too hard on Miller, from a standpoint of the fact they have to rely upon their repair centers evaluation more than the customers, especially since they can see "patterns" of behaviors with certain repair centers if they develop. If they keep seeing a lot of extra billed parts, delayed repairs, etc. or they have customers calling in to complain, it's easy enough to see if they are doing the right job over time and can work directly with the customer to get things resolved, and eventually weed out that repair center if they are not representing the company honestly or fairly. The fact is that they do have to draw a line somewhere, even if they have liberal policies, if they have a bean counter looking over their shoulder as many companies do these days. It does sound shady though on the repair center's end...so in this case it would be best to resolve it between Miller and the Customer...possibly on a cell phone while at the repair center standing in front of the repair supervisor or tech himself...
I remember a similar situation with a customer who called in to tell me a unit did not work. He said he had an electrician install the wiring for him in his house to code etc...It would not powerup at all. So, obviously from some of the details he gave, it was the power switch. Knowing that is not a frequent item of failure, I was trying to add things up. Then he told me that his electrician wanted to talk to me. As soon as he was on the phone the electrician started telling me that the unit was "underwired" for the amps it would draw and that he had rewired the unit correctly. I then asked him what gauge wire he had used, and it was well oversized for what was required, and likely would not easily fit into the powerswitch. I asked him if he was familiar with the NEC article 630. He had not ever heard of it, and said it didn't matter because he had done it for meeting code as the welder wire was undersized. I then explained to him the duty cycle derating etc by the NEC. He said he had never heard of such a thing...Anyway to make a long story short, the electrician had oversized the wire, and then over torqued the screws on the breaker switch on the rear of the unit, cracking the switch and breaking it. He could barely get the wires in to begin with... Once we got to the bottom of the story, the customer who was irate at us initially turned his anger toward his electrician in the end (who refused to even consider that he had made a mistake) after I explained it to customer himself and explained the issue over a series of calls. If memory serves correctly we sent him a new switch instead of sending it in after explaining that warranty repair doesn't usually cover this, but we'd be glad to send him the part at no charge if he wanted to fix it himself which he agreed to. IIRC, I think the electrician finally promised to install it himself and go back to using a smaller cord. Never heard from him again that I know of regarding the issue after we sent the switch.
Anyway, the thing to do, just like you'd do at a repair shop, always ask for the damaged parts BACK, if they aren't involved in a warranty repair. This helps assure that the center is at least being honest in what they did...and you can see for yourself the damage or perceived issue.