Transform - Hugs
First, I think what you felt when you saw her behind the counter is a perfectly normal reaction based on his behavior in the past. It is normal for you to internally question what could be going on.
Then it gets to the point where it is unhealthy. I've been there too!
So this triggered you to go look at his facebook page, my guess so you could prove to yourself that you were right, and they're sleeping together and what not. So there was no proof of that necessarily, but what you did see was enough to cause you pain and then look to see who SHE is friends with.
We always become "obsessed" with the OW.
When my XA told me he was going back to his ex, I wanted to know about her, she sings in her families band, and saw YouTube videos of her singing on stage..... I wanted to be sick. She's cute, and obviously very outgoing, and no matter all of the awful things he told me about her, when he wanted her back all I could see was that I wasn't good enough.
And now I am focusing on me and realizing that any man would be lucky to have me! Ok, I don't feel like that everyday, but I am getting there. And only able to get there AFTER I stopped looking for things about him or her. I don't want to know.
Correct me if I am wrong, but didn't you say before that you use facebook for work and other stuff so deleting it is not an option??
Ok, so if it something in your life that you HAVE to have....then why don't you block him? That way he is completely removed from existence on facebook and you dont' have to worry about seeing anything.
You think you want to know what is going on on his page....but you don't.
Spare yourself the constant pain, and do something to benefit your healthy recovery. Remove him. People have survived without facebook for years..... you'll be good if you can't see him on it. Trust me.