Me and my friend (A) share a mutual friend (L). Me and A speak to each other on the daily and we hang out often. Me and L haven't spoken in about a year and a half, with me sending a last message wishing her mother a Happy Mother's Day. For reference, L and I went to college with each other, and have befriended each other and even took a couple courses together. When me and my ex broke up, L helped me put a lot of things into perspective so that when my ex did come back asking to catch up eight months later, I really did not want to pursue anything further. L's mother is also close to my exes mother, and that's how I found out that my exes mother wasn't feeling well towards the end of the breakup--thankfully it was a minor issue, that did not require further medical intervention.
Anyways, flash forward to the past few months leading up to last week. L asks A about me, whether I'm seeing someone new, if I moved on from my ex, etc. L even asks A about how I'm doing too. A also mentioned that L expressed happiness when she basically mentioned that she was the one who caused my ex and I to break up. A doesn't say much, it's usually "idk" to these questions that L asks. I don't appreciate L wanting to know everything about me, when if she wanted to, she could just message me. I feel like she knows something about me, but I don't care to know. I don't know how to deal with her if I see her in person, do I stay civil?
I'd pretend all was okay and that I didn't know any of the stuff you just stated. Let her continue to think you're in the dark, and simply watch her to see what she's up to next. Don't tell her anything true about your life either. You can make up good stories about new men if you want to just to see her rxn.