Being jealous "of" something wouldn't really be used in this context. You would just be *feeling* jealous or *be* jealous because your friend is flirting with your girlfriend. Jealousy when it comes to relationships, love, etc. Isn't the same as being envious (which is a synonym of being jealous *of*). I'm jealous of my brother because me makes a lot of money. I'm jealous of John because he has the day off work I feel jealous when my girlfriend flirts with others. He is very jealous with his girlfriend. (He gets angry/upset when his girlfriend does certain things) He gets jealous when his friend flirts with his gf.

“I am feeling jealous because my friend is flirting with the girl I like.” This is the correct expression. In this situation, it is not correct to say you’re jealous of the girl. You’re jealous of your friends conversations/friendship with the girl. First state your emotion (I am feeling jealous) then state the reason (because my friend is flirting with the girl I like)

I think if you swapped ''that girl'' and ''my friend'' around it would make more sense, because you're jealous of your friend because he's talking to the girl, not jealous of the girl herself.