Assuming the scenario you give where the parent comes out and already has an established family, probably the same as if they told me they were getting a divorce and then some honestly. You know a divorce is most likely coming so that's a given. The rest to process on top of a divorce would be pretty difficult, you're talking about someone who has decided to change a major facet of their life that affects many things including their loved ones. I actually know of a man who did this exact thing and although his extended family eventually accepted him in his new lifestyle, and eventual partner, his immediate family (wife and kids) caught the worst of it and they pretty well fell apart.
As far as advice goes, I'd say just be there for your friend as I'm sure they will be having a very emotional time for a while. Don't give advice or opinions unless they ask you to, it's not your business, and just love them just the same. They will be craving a sense of normalcy (just as with any family that goes through a separation) and if you can provide that to them then I'm sure it will be appreciated. With their mom, just remember that sexuality does not define who a person is, it is merely one facet of someone's life, a secondary characteristic if you will. She will be the same person, just choosing to live a different lifestyle. It's just a shame that this decision comes at a point when so many others are likely to be affected.
I hope they are able to pull through this, sounds like you are a good friend to them if you are asking how to support them during this time, just keep being you :)
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