Shitty best friend? wtf?

A few months ago I met a girl on a 2 day fieldtrip through my best friend. I fell in love with her after a while and a few days ago on our yearly field trip I was planning to tell her. But my best friend that knows I love her basically took her away from me all day long, they we're flirting, holding hands, laying down on eachother,etc. What should I tell him?