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.
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.