Maybe you could offer to take her son out and kind of be like a mentor to him,like a big brother. Do guy stuff. Since you and her kind of have a friendship anyhow, and she watches your daughter for you, this would give her the break she needs from him in order to be herself and I'm sure even if you took him out for for even two hours, she would be relieved and grateful. Do you know if the father to this or these child or children is involved? Even you just being there as a friend to her is supportive. Would she allow you to have her girls to have a sleep over with your daughter either at her house or yours. Sometimes when kids get together and entertain themselves, this is a break from the kids even if it is a few hours. I remember when I was a child, my best friends mom used to come and get us four children once a week, take us to her house to play with her four children so that we were all occupied so that she could clean her house with out being bugged by her children and my mom got some time for herself for a few hours a week. I never understood that concept until I had children. Now I do understand. I hope this helps you and friend out tremendously. Good luck. Maybe if the children got into some sports, this would help to calm them down because they'd be using energy they need to get rid of. Sports like martial arts teach self discipline and control and respect and obedience and responsibility. This may be a great idea.
Unless she asks you for help or your opinion, then I would stay out of it. She may take offense if you try to tell her how to handle her kid. But it sounds like she is talking to you about the situation and may be open to ideas.