This is my first post, although I've been a viewer of gransnet for a while, as i find it to be very reassuring & informative.
I'm not sure that I can offer much support to yourself & your friend, but I did want to let you know that my family has experienced a very similar situation in the last year. My sister left her husband of 31 years for a woman in January 2014. Their 4 adult children of 22, 24, 26 & 28 have been badly affected by the break up, & have all favoured their father since. I don't have a relationship with my sister, and although I did speak with her about what had happened a year ago, I now only know how things are going through my mum, who updates from time to time. The one thing thats's obvious to me is, even though the children are adults, it has been such a shock for them that everything they knew & had as a family has completely gone. One of the boys will not have anything to do with his mum & is refusing to let her have contact with his daughter, her only grandchild. One son promises to meet her & doesn't show up. The eldest son was in touch initially, but now makes excuses in order to avoid her. Her daughter is a bit hit & miss, up & down, etc.
I think that your friends must ensure that they sit down as a family & at least try to talk about what's happened, & I would imagine that counseling with Relate might help them to get through the very bad times.
I wish you well & hope that your poor friend is ok very soon. Best of luck to you.