well... firstly, you have a boyfrend, right? if you like someone else above him, thass wrong. you need to figure this out first before the frend drama because the frend is just jelous which is less important than cheating ur bf's heart.
after you get that figured out,
gawr... well this guy already said he not interested right? so just forget it. of course your frend is going to be jelous... she should be mad right now. i mean... you already have a BF and on top of that, you, being her best frend, tell the other guy that you like him. did she tell him also that she liked him? if she didnt, that gives her more of a reason to get mad because it's like... if i was her, i would feel betrayed. just talk it out with her... if she cant cool it down, then somethings really wrong in your frendship.

but whatever you do, get over that guy. you shouldnt have to struggle in your bond with your best frend just for some irrelevant boy in your life

hope this is useful!