I casually meet with her ex but she doesn't know.

I’ve liked my best friend’s boyfriend before they even became an item but because he was playing hard to get I, I got involved with his cousin instead. Now that all these relationships are past, my best friend is not over her ex. I casually meet with the ex but she doesn’t know, not that its a secret or anything, so he kissed me this other day and I feel so bad…am I a bad friend for getting kissed by the ex?? And she doesn’t have to know right??