NO guarantee will cover damage caused by "misuse" by the user full stop, to leave something in storage for an extended period without inhibiting it against corrosion is a form of misuse.

If the rifle was returned with the instruction to "fix it" & the reg was fubar then replacing the part is the instructed repair, if the damage was caused by the rifle being "misused" by virtue of incorrect long-term storage, then of course the new part will be charged.

If you want to be asked before any repair is carried out, then you instruct "to investigate the fault & contact the owner to discuss any cost of repair or options"

Now I don't know what instruction was given when it was sent back, I would hope that the owner kept a copy of the letter giving the instruction & therefore has proof of what he asked to be done & whether his instruction was complied with.