We were at a birthday party for a friend of DS (turning 4) this afternoon at a softplay/little world place. They play for an hour then have tea and cake.
There's 1 particular ride on toy that all the kids want to play on. So they're taking turns. DS has been waiting about 10 mins. It's his turn and another child (older sibling, aged 5) snatches the toy and starts riding it. DS is upset. I look at the mother who rolls her eyes, shrugs and laughs.
DS comes over. I say "never mind, let's let Freddie have a turn and we can play something else. We'll make sure you get a turn". 15 mins later and Freddie is still riding the bike (DS has asked to have a turn several times and been ignored) and we get given a 5 minute warning for tea. Freddie starts taunting DS - riding the bike over to him, getting off then jumping back on when DS tries to get on. I look again at the mother who is laughing. I walk over and say to Freddie "that's not kind. It's time for DS to have his turn now, get off please". Which he does.
Mother then comes up to me and says "I don't need you parenting my kids thanks". To which I reply "well I thought someone needed to". She's spitting nails and storms away.
YABU - helicopter parent, let DS deal with situations himself
YANBU - you gave her every opportunity to step in on unacceptable behaviour, perfectly fine to make sure DS gets his turn