Thank you for your question. Since no one else has picked this up, I will step in.
It is nearly impossible to tell what is up without something in writing detailing what the charges are and what they are for. I also cannot get into too much detail because the Terms of Service here do not allow me to practice law and give you legal advice for your specific situation.
But I have turned a wrench a LOT with my own vehicles, after "graduating" from helping my Dad for years.
Legally, I think your reference to "negating" a debt goes only as far as the dollars go. Warranty returns are nice, but if the customer does not also take a replacement unit, there is the possibility of a "restocking" fee when the retailer puts it back on the shelf instead of in the customer's hands. Sounds like the replacement part was ordered from somewhere else, so this sounds possible. Now I don't myself "believe in" restocking fees, especially when more than about 5%, but that's what the industry done.
Legally, the debt being connected to a relative is completely irrelevant.
Fact investigation is key in consumer disputes like this. You can't tell anything until you get it in writing. Text messages sometimes don't count, and when dealing with a "merchant" under the Uniform Commercial Code, we live and die by what's on the written invoice(s).
But putting something in writing does not make it true. Some charges are mistakes, and some are close to being made-up garbage.
So you have to see the invoice and what the charges are to begin negotiations.
This concludes from your question that the replacement part was ordered by the shop performing the repairs and a charge for it appeared on their bill:
1. No one legally should pay for the same thing twice (not the same as buying one, breaking it with no warranty coverage, and needing to buy a new one--those are two different things paid for and received.)
2. No one legally should get to charge money twice for the same thing.
3. No two "merchants" should be able to charge the same customer, separately, for the exact same item delivered for that customer's use. See number 1. above.
Unfortunately, the law and courts are a blunt, clumsy and time-consuming tool for any dispute less than $10,000 in my opinion. Small claims court it a little bit better, but you must have all your evidence in hand before showing up, AND be prepared to disprove any fabricated evidence the other person brings that day--but you don't get to see their evidence first. But before doing that, you need to look at your paperwork and see if you agreed to mediation or arbitration before going to court. THAT is a very common thing printed on the paperwork that SMART businesses have for their customers.