Post event, it's a difficult one - as you've said yourself, you don't want to appear like an interfering loon pjh981's idea is a good one ^

What I've done in similar-ish situations before is to hand the Purser/CSD/nearest crew member my business card, such that if they do get a complaint, they've got an easyish means of allowing BA to contact someone to provide some balance (and know that the passenger concerned is willing to be contacted).