It seems that no matter how hard she tries to get along, communication eventually breaks down.
Then for a while there's a "honeymoon" period. They'll make up. And apologize. They'll spend time together. But eventually, it all unravels.
I suggest to the mother that she draw boundaries. Let the daughter know that she won't tolerate certain things.
After all, we teach others how to treat us. If our communication starts out clearly defined then there won't be as many issues.
The problem with these two, though, is that they have a 20+ year history. It's always been up and down for them. The daughter will be okay for awhile, then her sense of entitlement kicks in. After that things get bumpy.
And it started when the daughter was quite young. She cast herself in a victim role because her father was out of the picture. And she tried to make her mother the scapegoat - a role the mother accepted for a while.
While there's hope for these two, they must redefine their communication. What's in the past should remain in the past. If they still love each other they can make that the basis for a new beginning.
Life's too short to waste it fighting with our loved ones.