@shockers it was only going on for a few weeks before she found out.
@touchingcloth14 he hasn't been saying anything nasty about mum to the kids, which is one thing I suppose but he hasn't done anything to accept the blame either and hasn't really corrected what the GPs have been saying.
Thank you all for your replies and your support for my friend. She's quite a private person so was a bit upset with me when another friend told her I'd posted about her on MN, so I don't think she'll be signing up for an account but I have made the suggestion to her.
Things kicked off a bit at the weekend. I hope you're sitting down - it's a bit of a shocker.
Friday night - my friend was upstairs having a bath after a very long day at work whilst me and the girls were cooking dinner. (We had an evening of both Dirty Dancing films, wine, chocolate cake and, depending how drunk they got, Magic Mike XXL planned) (DC were with their dad). Her mobile starts buzzing on the dining room table. It's the sister. Cue much bitching between us about the skank girl. My friend comes downstairs just as the phone starts ringing AGAIN. Against our advice she answers it. Sister and the ExH have had a row - she thinks he's cheating because of some rumours that I can neither confirm nor deny that me and the girls were involved in. 
Because my friend is the kindest person in the world she listens to her sister blub for about 10 minutes before we wrestle the phone off her and tell the sister that she made her own sodding bed and she can bloody well lie in it. The evening continues after pouring too much wine in my friend's glass.
Saturday we (the friends) all go out with DHs and DCs and my friend's DCs seem pretty much back to normal ('look at me mummy!', 'over here mummy!', 'can you carry me mummy?' etc etc, which was lovely. The first time we'd really seen her smile in weeks.) Hope shimmers on the horizon.
Sunday - there's a party for my friend's father's mother (still with me?
) at her care home/residential home. Everyone is clapping along with 1940's drag queen act and having a great time singing war songs etc, when from inside an argument is suddenly heard. Sister is screaming at ExH and he's doing his best to keep her quiet. My friend's daughter gets involved because auntie is throwing stuff at her daddy. Auntie/sister swears at her
. To his credit - and this is probably the only thing - ExH gathers up his daughter in his arms and tells the sister that he won't accept that sort of behaviour towards his children and promptly removes a crying daughter from the situation, but daughter promptly wants mummy and wraps herself around my friend. Little brother is also now crying and clings to mummy's legs. GPs hurry over to try and hush up the situation. Everyone at the party is watching in fascination when in sweeps Gran - home from Europe as a surprise for other grandmother's birthday - and demands to know what is going on.

She removes the family members (friend, sister, ExH, GPs) into a room and the story all comes out. When they exit the room my friend's mother is looking very shamefaced after a proper dressing down from her mother.
GPs are now falling over themselves to apologise and it seems the spell has been broken with the DCs and they're back to normal. Gran has told my friend that she's sorting everything. It's not over yet but it's looking a darn sight better than last week, thank God. 