Idk what to do 😵

My best friend (i will use the name kay) has been dating her best friend for months( I will name him jack) and someone in our friend circle last year and while kay was getting lunch that girl (I will name Ally) told me directly that jack was texting other girls twice. I know for a fact that was a straight up lie because kay looked on jacks phone once (As a joke) and she would have noticed somethong wrong, plus jack is not that kind of guy. I never ended up telling Kay and Ally ended up spreading rumors about me two months ago. This has been eating up my heart and I feel more and more guilty of not telling her each day. I just dont want Kay hanging out with Ally if this is how manipulative Ally is. Should I tell Kay after all this time? I feel like she will get very mad at me 😓 And she has a right to be but I dont want to screw things up and cause more drama. btw it has taken me awhile to work up the courage to say this , I dont have anyone to talk to this about THANK YOU FOR TAKING YOUR PRECIOUS TIME READING THIS, IT MEANS ALOT 😁