Path20, I can understand that you are worried about your friend, but as some have already mentioned, you are not in full possession of the facts.
By what you've said, there have been ongoing problems for many years. This situation didn't arise over night, and just because your friend is lovely to you, doesn't mean that she's treated both her children in the same way. Sometimes, people only let you see what they want you to see. You really don't know what actually happened. Your friend may have been awful to her children, and might still be, given half a chance. I'm not saying that's the case, but you just don't know.
You say that the children only stayed with their father because he has money. You don't know that for certain, and they may have stayed for other reasons.
The fact that the son hasn't had contact for 6 years, and the daughter is only having minimal contact suggests that your friend may not have behaved in the way she should have done. It actually sounds as if the daughter is trying to maintain some kind of contact with your friend (albeit minimal), but is finding it difficult and doesn't want to get too close. If you interfere, or the daughter finds out that you are calling her names such as 'selfish brat', it could end the relationship completely.
In regards to the phone call, you don't know what was said. Also, you say:
'She just told her what a wonderful day out she had just had and to text if she needed anything.'
Why didn't your friend text if she needed something? She has chosen not to text her daughter to tell her what she wants, and then complains when she doesn't get what she wants, e.g. a visit.
It sounds as if your friend isn't being entirely truthful, or how she remembers things might not be as others remember them. I think it's best that you tread carefully in regards to this situation, and not let your obvious care and concern for your friend cloud your judgement. These things are rarely straightforward.