Just think about what you have "lost" here :
A man who is a cheat
A man who lies to people he is close to
A man who shows signs of depression
A man who doesn't have much money (he can't be well off, or you wouldn't have fallen for his sob story about finances)
A man who is at best insensitive, at worst cruel - by uploading pic to FB on Christmas day, knowing you would see it
A man who probably has (or soon will have) ED
A man who doesn't have a house (?)
You are grieving for who you thought he was, but the reality of him is all those things I've listed above.
He's 55 and he doesn't have his shit together I say that as a 55 year old woman!
No way in hell will this fling last. Let's not forget, he has cheated on her already (with you), so he isn't in love with her!
When he's 60 and can no longer shag, she will only be 35-40 and about to enter her sexual peak. She will be dumping him for someone younger.
You really should send her screenshots of the messages he sent to you, whilst he was seeing you both. I have no idea why you wouldn't? You're being a mug in that regard. I would have even posted on the FB photo - "shame he was shagging us both from March to June" or something of that ilk.
You owe him nothing, do not take this lying down Send her all the proof, and then pour yourself a nice glass of wine and let the shit unfold.