The key thing here, for me, is how you let them back on. Absolutely 100% grab an inhaler with no thought of a card. Come back on to the FOP afterwards without permission - that's then a booking, regardless of what medical reason you went off for. In the OP's scenario, at this age group, I would be wanting a word with the player prior to them coming back on - double-checking they are OK and letting them know if they need it again to either loudly let me know or just go and grab the inhaler (I had a similar scenario a number of weeks ago - U15 player about as far away from coaches as could be, hands on knees ignoring play around him; that was a shout for the coach to come on).