OK, he's evil but The Hound rules.
Lord Frey may have killed Robb, Catelynn and Talisa but they can't possibly kill the entire army Robb had parked near The Twins. I imagine there will be retribution for this when the troops get word.
They all died, their army was significantly smaller, which was why they needed Frey's troops. That, along with how they were caught by surprise means they were wiped out. Might have even had the help of Bolton's troops as well.
Wow, my typing was a mess, sorry for that, I think my keyboard is on it's last legs as it is constantly leaving out letters, just getting tired of re-editing with every post.
To answer your question, yes it was in the book, but I'm not sure if it was merely mentioned or shown. Been only a year or so since I read it, but I read all the books quickly and one after another, so I have some recollection issues. Whether or not it makes it into the series will be seen next week.
It was only mentioned, not shown