Sexual assault

So today I found out that my mum was sexually assaulted on a bus. It was very hard to hear and I feel so sorry and I want to do something for her, but I don't know how. She said that she fell asleep on the bus and the man beside her (who, mind you, was only about 30 and she is late 30's) was putting his hand in her pants when she woke up. She said that she completely froze, and since my mum doesn't speak the language here she couldn't do anything. My mum told me and my older sister this because we frequently take the buses here and I have always sat with random strangers and never thought twice about it. Now, I don't think I ever want to again. I'd rather walk. I always found it strange when a dude would sit right beside me when the seats behind me were empty, but I thought nothing about it.

Anyways. I just want to know what I can do to help her? I know nothing about this (as I am 15, and was raised very sheltered) and would like some advice, or simply just someone to listen, for once.