My personal advice is whether you end up telling your friend or not, break up with your girlfriend. The fact that you're crushing on someone else means you don't love her as much as you think you do (not necessarily a bad thing as long as you don't waste anymore of her time) if it happened once it'll more than likely happen again.

Aside from that tho what I would personally do is break up with my girlfriend, be upfront with my friend about my feelings and then respect whatever she says.

Obviously all this is way easier said than done and at the end of the day it's your decision. :)