Someone that I know is dating an actual prostitute, do you think I should tell him even though I don't want him to know about it?
Why don't you want him to know?
I don't want him to judge her based on that.
He might get into trouble by not knowing about this. You must tell him and let him make his own judgments.
I just don't know how to tell him that, I spent a long time with him.
You should just be open and honest with this person if you care about them at all. They deserve to know about the person that they're with, and if you withhold this information then you could be being a pretty poor friend.