Hmm, I'm sorry I'd be a bit peeved if someone who was kind enough to use his extinguisher on my plane then turned around and asked me to replace it. It costs a measly $40 bucks to refill. I just potentially lost thousands. Wait a week or two BEFORE bringing that up.

If I left my extinguisher in the car (this NEVER happens), then just exchange your spent one for my full one. That would be cool. You gotta keep flying. That's understood.

If you aren't a bro enough to help your buddy's airplane stop burning for $40 then don't volunteer your extinguisher. Be a TOOL and just watch your buddy's airplane burn.