I would like to see this feature added.

My suggestions would be:
- The player requesting a repair does not need dark matter and that the game simply charges them the same fee as the Mender NPC. This takes gil out of the game and does not cost the crafter anything except time.
- Repairs should not go above 100% (I'd be happy with the suggestion of 50% that someone made above, which would discourage people from relying on this)
- Repairing should of course, be entirely at the crafter's discretion. If someone comes into a dungeon with broken or almost broken gear then a kick is justified, but it would be nice to have the option to help people out.

This would be very helpful for statics learning fights and for dps players like me who would much prefer to repair someone's gear over having the group fold and going back to sitting in a queue for 20 minutes.