I think IG was right to place boundaries and reprimand her for appalling behaviour - but his wording and approach was clunky. He should have preempted it with 'I love you and always will, but right now a treat isn't appropriate and we can talk about why', he definitely should have suggested an alternative - making a 12 year old decide implies he isn't that bothered, and don't think he should have bought BW in as a reason not to do something. It upsets the kids and actually puts the girlfriend in an unfair position, reinforcing her status as the 'bad woman who stole Dad' etc.