In similar circumstances I've given my business card to the FA/GA and let them know I'm happy to write in now or just let me know later if any help is needed.

Often the bluster fades and the complaint never happens so I just feel that giving them the option not to alert 'management' unless needed was worthwhile.

Last occasion I did this was for a GA faced with a classic DYKWIA rant that she handled entirely professionally and politely with out ever raising her voice yet she was accused of being rude and unhelpful despite the fact that what she was being asked to do was neither possible nor permissible.

Amazing how many times people characterize a negative response to an unreasonable request as 'rude'