Just be there for her and TRY really hard not to slag him off.
My sisters husband is a complete selfish wanker. He's awful with money and in the last 4 months (after taking 7 years to clear his poor credit rating after being stupid with money) took out a credit card without her knowledge, maxed it out, then took out a loan to cover the credit card.. then just spent the loan.
Added to that, he was having an emotional affair with a woman from work.
Added to that, he used the paternity vouchers his work had given him on himself.
And he doesn't do anything round the house.
My sister kicked him out and lasted 4 days before she took him back 
I was there (and so were my other siblings) to give her loads of support with her daughter - we stayed with her, let her talk and all of us agreed we wouldn't say anything negative about him at all in case she took him back.
As she has taken him back, if we had said 'He's an idiot, if you take him back you're a moron', the worry was that she'd feel less inclined to come to us the NEXT TIME he does something stupid, which I am sure is only a matter of time - if your sister feels you judge her, she may not open up as much.
Don't get me wrong, I really, really wish she had thrown him out, divorced him and met someone who actually respected her and wasn't a complete and utter idiot - but she's got to make that decision in her own time.
I kept asking my sister 'How are you feeling?' and 'What's the best outcome you'd like? Do you think that's feasible?' and trying to stay as neutral as possible.
I agree with you - I think he's met someone else as well - perhaps the OW has decided she doesn't want your sisters Husband so he's 'decided to stay?'