Tonight, I was chatting to my best mates crush.
He asked for a hug, and I said 'okay'
Then he started telling dirty jokes and I went along with them, and he was kind of um... Flirting, and now I feel bad because I flirted back!
And I even told him I used to like him!
Help guys- what if he tells anyone!

If he's not attached to anyone.. crush or not.. it was harmless. Your choice to leave it be now or continue the flirting. Knowing your friend has a crush on him.. I'd walk away and just put it down to being friendly and nothing more.