Have you ever forgotten your gear when showing up at a gig? I know I haven't and I've been gigging since 1989. It's kinda hard playing music without your instrument, me thinks.Sorry, I'm not buying your high and mighty attitude toward people who make a mistake. If you are perfect, THEN and only then can you spout off about others- especially considering you have literally no idea what actually happened.
Personally, I wouldn't ask to borrow a cymbal, and I think it is bad form to do so, BUT...I wouldn't slam a guy who accidentally forgot something. I'd tell him that it's a bummer, but I prefer not to loan out my gear.
The comments slamming the guy for forgetting are ridiculous, I'm sorry to say. Slam him for asking, but not for forgetting.