My parent's marriage is falling apart. How would you handle this?

I left home a couple of years ago but my younger brother is still living at home. We have always had what seemed to be the perfect family. Until around 6 months ago... my mum started having an affair. It's been out in the open for a couple of months and my dad has forgiven her and asked her not to leave. For a... show more I left home a couple of years ago but my younger brother is still living at home. We have always had what seemed to be the perfect family. Until around 6 months ago... my mum started having an affair. It's been out in the open for a couple of months and my dad has forgiven her and asked her not to leave. For a while it looked like things were going to be OK...
But just after christmas, my mum gave me a letter explaining how she feels she has married the wrong man and cannot carry on her life as it is, and wants to leave my dad. Then she moved out. She's been looking so ill lately and has lost around a stone. At the time I felt obliged to let her go without interfering, as it is not my place to judge my parents' actions and I don't as a rule advise people who don't ask for it.
A couple of weeks on & my dad is inconsolable. He used to be my pillar of strength, now he calls me up in tears. He wants me to persuade mum to come home. I can't refuse to help my poor dad -
What should I do?
Update: Although many have advised me to stay neutral and let them fight it out (thanks for advice) my dad sees it like this: my mum leaving will affect the whole family and it is up to the whole family to hold itself together. We are all so close to each other, we cannot let one of us trip and fall without trying to pick... show more Although many have advised me to stay neutral and let them fight it out (thanks for advice) my dad sees it like this: my mum leaving will affect the whole family and it is up to the whole family to hold itself together. We are all so close to each other, we cannot let one of us trip and fall without trying to pick them back up. My brothers and I are active members of the family, not just passive recipients of the consequences of my parents' actions. My mum is depressed and not herself, we can't just sit back and let her go nuts and wreck her family. That's how he sees things anyway and a part of me agrees.
On the other hand, I'm only 21, I don't know how marriages work and I don't know how much damage I might cause by interfering. I can sympathise with how my mum is feeling trapped and hopeless as well, I don't want to damage our relationship by taking sides. But then - if they divorce and I did nothing to stop it, how can I forgive myself?
Aaargh.
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