<human>: She (29,F) and I (28,M) started seeing each other in June. We went out for a while and I made sure my intentions were clear. She said she didn't want to get into a relationship, that the timing was all wrong. She's moving away next fall for grad school, and a year ago she was almost married. I said ok, let's be friends. 

Fast forward to today when I see a post on r/relationship advice that she made asking advice about a guy. According to this post, around the same time we started seeing each other she considered herself in a relationship with another guy. I'm not really jealous about her seeing someone else, but I am a bit hurt that the reasons she gave me for not wanting to be in a relationship with me were completely false. 

Also, I'm a little upset to find out that I was the guy on the side. This same situation keeps popping up in my life. To be perfectly honest, I've been the guy on the side more times than I've had an actual relationship. I'm beginning to wonder if there's something wrong with my bull shit detector. 

The advice I need is how to handle this new information. My initial reaction is to say "fuck you bitch, I'm out." Because you know, trust issues. If I do want to keep her as a friend, should I keep this knowledge secret or should I own up that I know it? 

One further complication is that I'll probably be unable to completely avoid this girl, as my step mother loves her and regularly invites her and me to dinner. Yeah, awkward.

Provide a brief overview of the text.
<bot>: Girl lied to me about reasons for wanting to be in a relationship, and the truth was that there was someone else. Do I let her know that I found out, keep it secret, or burn that bridge?