Not sure how to play this so I'd welcome some thoughts ...
DS has been mates with this kid ( Flipper ) for years, they go to different schools but hang out together pretty regularly during the holidays. Flipper's Mum ( FM ) and I are friends and meet up with and without sprogs every few weeks - so far so good - but ( Here's where it gets a bit tricky ... )
Two weeks ago I bumped into FM who said she, Flipper and Flipper's Dad ( Her ex, who now lives abroad but bobs up from time to time ) were off to a local Steam Fair thing the following day and would DS like to come too. When I got home and asked DS he said he was up for it so I rang FM and we left it that she'd ring half an hour or so before they came round to pick DS up ( I was going to be at work ) so he could get ready.
When I got home that night it was to a glum DS who'd sat around all day waiting for a call that never came. I rang FM and she said Ooops-sorry-they-forgot, and that anyway Flipper and his Dad were busy taking photos of the steam engines and didn't want DS to be bored 
I've not been in touch since. I'm pissed off. DS is pissed off. Don't get me wrong - I'm queen of the social lie - if Flipper didn't want DS to come along because he and his Dad wanted to do some Manly Bonding while viewing the chuffers then all they had to do was ring up with some vaguely credible excuse ( Tummy bug / dead car / zombie apocalypse ) and everything would be peachy, but to invite the poor sod and then just forget and/or ignore him has really annoyed me.
What would you do?