I am not clear on whether they left the room specifically to talk about the tickling without the OP knowing that, or if they all left the room just because they were done eating, and then the tickling happened again.
With the leaving the room issue aside, with my closest friends who know my kids well, I would be appreciative. Both of my kids tend toward wildness and can be really persistent. I like for them to run up against reasonable limits that are enforced by someone other than me, because it reinforces that these are widespread expectations for their behavior, not just my
expectations. Also when one is not listening to me, and I am at the point of needing to escalate my own discipline with either a time-out or leaving a playdate or gathering, sometimes a word from a friend to my child about their behavior can turn it completely around and the child gets it and we can go on with the playdate or gathering.
With someone who doesn't know us well, I don't mind at all if they are speaking up for their own child in an informative and non-shaming tone (and I feel bad that they had to do it because I wasn't on top of it first); and I don't mind if someone we don't know sees me struggling with my kid and "speaks up for me" in a reasonable tone (ie "you should listen to your mom") because hearing that from another adult really
gets his attention and can be a big help (and I always say "thank you !!" over his head. )
The only time I don't want others disciplining my kid is when their expectations are overly rigid, their tone is shaming, and my child was not doing anything they had any way to know was not appropriate. Then it bugs me. But with my kids temperaments
, that is not usually the case.