Parts sharing happens a lot between airlines.
There is an old pic somewhere in the database of a BA
747 classic with an AA
nosecone, and Delta has for some time operated a system with a number of international airlines where they can pick parts from common stock.
Just two examples. For instance...
Delta had an arrangement with Britannia in the UK, so if a 767 went tech here then Britannia could source a part for them. Replacements and payments could then be sorted out later. There were other airlines within this internet-based inventory system which could locate a spare closest to the aircraft in need.
Lead me not into temptation, I can find my own way there...