What kind of things is she posting? And nah you have more class than to contact or speak up to some random girl. I think he has gone to far! And he has got used to you being there even though women are in his life. At least mine tries to hide it. Although I don’t know which way is worse ????
People who have to state they’re happy online are far from happy! That screams self esteem issues to me.
Are you watching to see if she says anything about him? I’d suggest trying not to look at it as it’s only going to wind you up.