I can't imagine anyone being on the internet in this day and age not knowing what a troll is.
There is, in fact, no reason why she did that. She could have stopped talking back to him. She could have left. She could have just laughed at him. All of which could have ended the situation, being new or stupid or even a combination of those. It's obvious she didn't like what he was saying, so I'm curious as to why she didn't try to avoid the situation or try to make it better.
As for an excuse, some people are just assholes.