I'm going to tell my roommate that I don't like the guy she's dating.
Why do you want to do that?
He's into loli porn and he has disgusting habits.
There is nothing wrong with being honest with your roommates. Just make sure that this will not cause things to get heated between you two down the road.
Ah... Maybe... I don't know.
If you two are friends, I am sure she will understand your opinions on the guy. What she does with the information is her business and you have to respect that.