I totally agree that lancet devices should only be used by one person, however I think that your daughter did the right and sensible thing in offering him the use of hers. After all, if it was the other way round and your daughters cap had gone astray (for whatever reason) would you not have been gratefull to the other child for offering the use of his finger pricker untill the cap was found.
Kids being what they are this sort of thing can happen to any of them. We don't know why the cap was missing but lets hope it was a one off.
I think you have a very caring and sensible daughter there.
Harry