D..
I'd talk to a lawyer in your mom's area and find out what the legal ramifications are. She needs to know just how much her rear end will be in a sling if one of these kids drives and is involved in a wreck or even if they just get a DUI. She needs to know what she needs to do if she finds out they are drinking in her apartment as well. Bottom line, she needs to learn HOW to protect herself legally. Insurance will probably not help her if she allows underage drinking in her apartment. And her daughter WILL sell her out if something happens, telling authorities that your mom has known about this all along.
Forewarned is forearmed. Get armed...
Dawn