It's not fair to the kids... The mom is bringing this on by her actions and she will have to deal with the results...
Yes, she is doing more damage to her relationship with the kids than she realizes...
My ex went through the same thing... She took off for a year after I paid her off in the divorce... She would go to her boyfriend's house for days and not take care of the kids... Her parents stepped up...
It broke my heart to see her tear the boys hearts up by telling them "I'll be home in a while..." The kids would wait and wait and wait for her to show up, just to be broken hearted when she didn't... She would show up 3-4 days later... She did this to them over, and over, and over...
Now she doesn't understand why the oldest boy doesn't answer her texts and calls most of the time... He remembers when she abandoned them and has some "issues" with her because of it... He says, "She didn't want to have anything to do with me when I was younger, so why does she want to be in my life now???"
Just try to keep the kids occupied and out of the middle of this as much as you can to minimize the effect on them...