This girl flirts with my crush, help?

OK so i like this boy and he likes me too, he told told me that. the thing is he is too shy and shy to ask me out he is a shy sweet kid.the thing is this girl who used to be my best friend kept on flirting with him she knows i like him shes trying to make me jelous.i dont blame him at all shes the one being flirty and he listens.so im not asking u to tell me how to stop her from flirting with him my ques is what should i exactly do to show her i dont care what shes doing and im not getting jelous and make herself look like an idiot/fool.plz help shes gettn on my nerve.she thinks shes all that.