What to do…

You shouldn't go after him. Just because he's taking a break with his girlfriend doesn't mean he's available. If you try something, he might turn you down and you could also lose your friendship with that girl. Don't do anything. Even if he would break up with his girlfriend, that doesn't mean that he wants a new relationship right away. People have feelings. Once they break up, he'll not be waiting to have someone hitting on him right away. Just because they're taking a break doesn't mean they'll break up. There's also more to a person than just the physical part.

Life is supposed to be messy. Making risky decisions is how we learn and grow. You think he's hot, so go for it! You used the word "kinda" to describe your friendship with this girl. Obviously you're not best friends, so who cares if you piss her off? What if you two hit it off and you fall in love? He could end up being your long term boyfriend. Or, it could all go tits up and you could lose them both. But if you sit on the sidelines, second guessing the decisions... you'll just end up with regrets and missed opportunities.

I think you should just go for it. It doesn't sound like you're good friends with the girl. Also if they are taking a "break" that means things aren't working out for them anyway so you wont't ruin their relationship. Just tell him how you feel and stop wondering.