No, you should not get involved, your daughter must deal with this, as she is actually being bullied by her own daughter and must show some strength. My daughter was awful aged 13 to 15, worse than her older brothers, so I know the scenario. Your daughter is enabling her daughter to behave like this by continuing to treat her and give her what she wants. We all love our kids unconditionally and want them to be happy but there are basic lessons in life that she needs to learn, despite going thro teenage hormones etc She knows right from wrong but is choosing to ignore it, so she needs some tough love. If she won't talk, she should send her daughter an email spelling out how it's going to be from here onwards, bad language and behaviour will have consequences and pocket money, clothes money etc will be withdrawn. She should also tell her just how much pain she is causing. I have 3 kids, 5 grandkids, have worked with young offenders for 15 years and in a senior school for 5. It's very difficult, but to continually give in to them just exacerbates issues and teaches them nothing. Decency and manners are life skills we need wherever we go.