Even prepared people forget stuff. I would start with don't sweat the small stuff, meaning the 10 essentials is pretty much acknowledged as, "You are a HUGE idiot for not carrying this on you". You're saying this person is somewhat of an outdoorsman, so by reminding them they have a decent history of adventure their experience is wide but shallow, attack their level of preparedness compared to their level of experience.

Sarcasm I think would work well here. "Wow, you got a lot of experience, too bad if you get into trouble and have less prepared people with you, you're all gonna die b/c you let your ego control your gear." "Gee nice to know, you've learned nothing about being prepared while spending so much time outdoors. how do you start a fire w/o a bic?" and on and on and on.