Thanks for all the replies 
I had considered telling her, then did start to worry about possible revenge from him...
Then I suddenly remembered the twist which is that I have always know that she is married (unhappily, allegedly). Which means she might have known about me, and lied to me in reply... And, obviously, that in any case she won't mind he cheats... (Which is making my question irrelevant...)
His answer to my texts was a long email denying the things on fb (which have since stopped) were about him, (indeed he wasn't named - it made sense it should be him, timewise, etc, even down to a break when he was on hols with me.....) and telling me we need to talk about where we are going (which, so far, he had not wanted to do...) ( which he said we should do when he comes to visit on Wednesday, and he said he is looking forward to, hinting to positive things).
I was v happy seeing his email yesterday but im beginning to be doubtful again and feel like saying goodbye and not even wait until Wednesday; I now feel I can't trust him, but can't put my finger on it. He has lied before... He has a diagnosed communication problem... (=he's not the master in his own house unfortunately, and I don't think I can expect emotional literacy from him...) And can be quite arrogant... But then I was always 'undemonstrative' as he says, and withheld a lot (fear of getting hurt too.) until v recently...
Not sure anyone will have anything to say to that; maybe if anyone could could help me ask myself the right questions?