My mother is being sued for a car accident and I am the witness, what do I do?

Around two years ago, my mother and I were driving to my sisters house. We were going straight for a while now and in an intersection, out of the corner of my eye I see a car that passed a the stop sign and I told my mom to stop but it was too late. She tried to brake but the driver was already in front of us. Our front hit the drivers door and left front area. No air bags for either of us , just damages to the cars. The driver is a teenager, like me but I suspect he was on his phone and not paying attention, although I cannot prove this. My mother has never gotten into an accident before and doesn’t know English so we really didn’t know what to do. I just knew I had to call the police and report the accident. No one took pictures of the scene, since for everyone there it was there first accident so we were oblivious. The police report was also lacking valuable information from what I’ve been told. It’s been two years and my mother got a letter saying she’s being sued for damages. Help