When my friend's husband walked out on her for another woman, after many years of marriage, I found that the only thing I could really do to help was to listen. She needed, desperately, to talk about it. She never expressed any hope he would return - I don't think she wanted him back, after the way he had treated her - but she did become seriously depressed for quite a long time, both before and after the divorce was finalised. She coped well with the practical stuff, like selling the house and moving to be closer to her sons, but the emotional damage was really bad. All you can really do is listen and understand.
But, on a practical note, if the divorce goes ahead you could help her to find a really good solicitor. My friend's was brilliant and made sure she got every single thing she was entitled to.