This happens how frequently? I have a hard time buying this is an epidemic far and wide ... and the OP wants to stop development on other more important stuff in order to develop something that will fix a problem they have probably only experienced once in their life and likely will not experience again.

While finding a solution to this is not needed, I do agree that we should be able to repair other people's gear. It should be via request from the owner of the gear (like it was in 1.x). Requester must offer either the dark matter (if they have it) or the gil (locked to value of dark matter). If gil is offered and the crafter has the dark matter then they get the gil. If gil is offered and the crafter does not have the dark matter then they get and lose nothing. Can be done in open world or instances.