I read your post several times and I'm still not understanding why you felt complelled to do this work for free. I'm sure I'm missing something here but I just can't find it.
It's called customer service!
The customer doesn't want to hear my problems, he just wants his car fixed and having had it fixed only two months ago by my company he certainly isn't expecting a bill.
Some background info:
The dealership is part of MB(UK)ltd, not a franchise.
Many years ago I used to work there as a tech and the people that run the service department are the people that I used to work with 25 years ago.
They are only a couple of miles from me and they don't like how much business they lose to me.
My relationship on a day to day business is exclusively with the parts department with whom I spend around £125000 per year. The parts department treat me well and I suspect that they will refund me the cost of the parts when I tell them what's going on.
Other dealerships are constantly courting my business but none are so close and cannot offer the rapid delivery three times per day that I currently rely on.
The last time this happened I took the trouble to protest in writing to head office but it was a waste of time.
I just view it as a cost of doing business. After all, the worse they treat their customers the more of them come to me!