Parents divorce

In May (on my little brothers birthday) my mum decided to up and leave. Long story, basics are she found another guy, got engaged to him before her and my Dad had spoke about divorce properly, and now only see's my younger siblings once a week - after being told by my Dad she still needs to see us.

I live 12/13 miles away from my Dad, but see him at least twice a week (youngest sister still goes to school where I live - story for a different time) and on Saturday it was his birthday. Most of us turned up at my older sisters for a small party for him, but because my Mum wasn't there, he left in tears after 20 minutes.

Does anyone have any advice on how I can help my Dad please?

I'd love for things to get somewhat better before Christmas, as half the family are at mine this year, hopefully including my Dad.