When should the line be drawn when you have a teenager that's unruly.?

I want to know,my mom friend has a 17 year old that slept with the neighbors husband.This is not the 1st time little Ms.Thang has done this.That's why my mom friend always haf to move.So where do you draw the line?I told her stick foot up her azz.My mom friend works 2 jobs to ensure her azz has everyhting she wants and needs,but I told my mom that's the problem.Any suggestions?