I don't know if she is guilty of blackmail, but according to a R4 discussion the other day, apparently why celebs go down the superinjunction route when being blackmailed is because if they go to the police the cops can't keep their traps shut and it will be on page 1 of The Sun the next day. 
I think he's either being poorly advised or going against the advice of his lawyers though when he talks about suing Twitter or individual tweeps. Doesn't he realise what a massive chump this makes him look, more than for having an affair then trying to keep it secret, in my eyes. Plus the obvious question of on what basis he can sue them as they were just repeating stuff that was in the public domain anyway, and the wisdom of a fabuluously rich footballer suiing a member of the public...what's he going to do, take their house off them? Oh well done there. Also FYI I read about the affair on Mumsnet BEFORE someone set up the superinjunction account on Twitter...
Coming back to the thread title I don't see that Imogen Thomas has any cause of action against he-who-must-not-be-named though.