I was going to suggest a group so your children aren't the only ones for her to focus on. Could you suggest a playgroup? In my area there are a couple of community playgroups facillitated by OT's and speech therapists that are open to everyone. This way you could still see your friend, there would be someone facillitating play which may help your ffriend while she is coping with the new bub and they may also have some stratagies for dealing with any unwanted behaviours? It may also open the possibility for more socialising for your friend, even if you don't go all the time

Sounds like you are doing your best to be a great friend xx