My baby father's best friend(female) kissed me! What should I do !?

His best friend has told me she likes me and has been telling me when my baby father cheating so I never told him bout her liking me so I could find out more.. Anyways last night him and his brother went out and their was a little flirtatious talk now I think bout it but she just came up behind me and kept kissing me! I pushed her away a bit and said what if he comes back.. But me and baby dad been so good lately and he actually been loyal and for once he's starting putting in an effort, I feel so bad even though it's not my fault!! I dunno whether to tell him(he hasn't been honest in the past) but then I feel like I should be..  I don't wanna ruin anything what should I do!?