For the most part, a guy with any sense of morals, should give them back at the close of an event.
We have "loaner gear" that is basic: hat, shirt, coat, pants, canteen, blanket and cup. We are artillery, so no need for weapons/leathers.
If he is a friend of a unit member, the unit member usually reminds him that it is loaner gear, thus foster the knowledge that it should be returned.
However, if you are looking for a "rule" for your unit, I would specify that gear needs to be returned at the end of an event, and that gear can be loaned out for future events, until the individual can purchase his own. If a guy is a decent fellow with enthusiasm and the cash means, it won't take long for him to start accumulating the things that he needs.