This isn't a crime. Sorry but it just isn't.
WhateverTrevor I appreciate that the police aren't always fab at applying the law, but that doesn't stop something being against it. While the more serious offence is indeed where threats are made (s4), a course of action which the perpetrator "knows or ought to know amounts to harassment" is sufficient by itself to constitute an offence (s2) under the Prevention of Harassment Act 1997. It's both a civil tort and a crime, in fact. Having said that, I also doubt the police would do anything at this stage when the person can be blocked so easily via Facebook settings (which would be the easiest and most dignified route right now) - if she tries an alternate route to communicate, though, which on current form sadly can't be ruled out, then they might well be willing to get involved. It would depend on who she spoke to, but police are human, and the very clear-cut, obvious unfairness of this might indeed make some nice copper (and I've met a few) willing to have a sharp word. Which would in all likelihood be enough. So OP, block and ignore, but do so in the knowledge that any attempts to hurt you via any other communications are indeed against the law, and can be reported according.
Hopefully she won't go there - but God, she's a stupid little brat. What an idiot your H is, to have got embroiled with such Jeremy Kyle fodder. I am very, very adamant as a rule that people should NOT blame the OW, because she doesn't know you and made no promises to you, so the betrayer is the spouse - but in this instance she's a just nasty piece of work. Someone who could be that breathtakingly, calculatedly vicious to someone they've done something so cruel to to begin with is, basically, scum. It's sad he has such low standards, given she is a worthless guttersnipe, but at least you don't need to worry what she may have that you don't. It's already apparent - no conscience whatsoever, so no qualms about hurting anyone and thus no obstacles to jumping into bed with a husband and father. That's it. Yippee for her.
I'm afraid it does sound like an STI check is essential, but they will be lovely to you. You're not the one here to have any grounds for embarassment. As to people at their work laughing at you - oh, please. They'll be disgusted by them, not you. Any laughter will be at how cheap and pathetic the pair of them are. You're the only one in this sordid little mess who has dignity and clean hands.
I'd make him bog off for a while so you can do some thinking, and focus on yourself and your son with clear headspace. No need to make any big decisions yet. I'm so sorry other people's shitty behaviour is impacting you so horribly - life can be brutally unfair.