How do I support my partner through this mess?

[NameChanger5000]

Have NC'd in case anyone involved is reading (unlikely).

Came home yesterday to find DH on a long call to his parents. His mum phoned him "so he could her how his dad was talking to her". Mum was hysterical. Dad completely shut down. During the call DH found out that his dad had been having an affair for the last 29 (!) years, apparently ended last year. This is most of DH's life. His dad and mum are now arguing daily - he gets verbally abusive and demeans her, she reacts by throwing things, crying, shouting. Most recently she woke him in the small hours to ask for more info about the other woman, they argued for hours, he drove off, vanished for a day, and so on.

His mum has form for phoning so DH "can hear what his father has to say".

DH is, understandably, incredibly shaken and sad. I don't think it'd sunk in yesterday, but this morning it did, big time. I told him that IMO he shouldn't be acting as referee/mediator between them - it won't work and he'll find out things he shouldn't.

But how the hell do I help him through this? He's gotten up to go to work as normal but he's incredibly down and looking very lost. No idea what to do / suggest.

Parents live very, very far away if that makes a difference.

Phew, difficult one.
Well the fact that his dad has just changed his understanding of his own life and his parents' marriage will need time to sink in. I'd watch out for some clingyness or neediness or strange behaviour where he wonders if your life is also a lie...
Then there's the question of his actual parents ongoing relationship.
Sounds like his mother needs a man to validate things and protect her? As in, man, listen to this other man and then protect me from him....

Don't answer the phone.

You listen when he wants to talk, you support him, you don't judge or tell him its nothing to do with him, its his family, his parents its his right to deal with it how he sees fit. If I was him Id be telling my mum to come stay for a while.

chic I've told him that as far as I'm concerned she's very welcome here if he wants to invite her. She's never left their home country for various reasons so I don't think it's a step she'd take easily but he can at least offer.

When he asked me last night I said he should tell them that they need to have these conversations between themselves - that as their son nothing good will come of him trying to mediate. That he's there for them and wants to talk, but not when they are shouting at one another. He agreed, but I don't know how realistic a plan it is.

29 years!? That's horrendous - your poor MIL and your poor DH!

It's quite possible that he might grieve, in a way - grieve for who he thought his father was and for the loss of normality. That sounds a bit extreme, but I imagine a revelation of that magnitude can really change, as something says, your understanding of your own life. Be gentle with him

What you said last night sounds good, but he'll need to maintain those boundaries, which will be hard when his mother is so distressed. So I think gently holding him to that plan would be useful.