I'm going to tell my friend that he shouldn't be so worked up over his ex
Are you worried that he's thinking about his ex too much?
He's my friend and I believe he deserves to move on, because that girl was bad for him.
It's good to want to help your friend to move on.  However, he may still have feelings for his ex, so you shouldn't risk badmouthing her too harshly, or this could hurt his feelings as well.
You're right, I should tread carefully.
Definitely be careful with it, and help him move on instead of badmouthing his ex to him. Especially if it could get back to her and then both of them would be hurt.