too much drama for my judgement.

My friend jake went out with a girl named sarah a year and a half ago but never really got over her. I liked her if not loved her only a year ago, but refrained from acting at all on it because I knew it would hurt my best friend jake. So I barely talked at all to sarah for almost a year. I thought if I did this I could start talking to her later once my best friend was totally over her. Now, a year later, I started talking to her again and we both started liking each other. My friend jake didn’t really know. It was all going as planned until jake randomly went to her house and asked her out, without any notification to me or her and without talking to her much for almost a year, they even went to different schools. This drastically screwed me over, because it is now clear that he is still not over her. I hungout alone with sarah four days ago and kissed her, but jake doesn’t know. I want to tell him somehow but now that he asked her out I dont know how I can. I recently asked him if he was over her, and he said, I dunno, I guess I am but I don’t know. that kind of sounds like a yes. what do I do?! I have a date with sarah in five days. I postponed it so I could talk to jake, but I don’t know what to say. what do I do?!