I think he has a right to know yes. Agreed completely that the long term deception is the worst. I also think that for many people that living with the guilt can be as destructive as the actual infidelity, though only the OP knows if that's the case for her or if she can put it behind her.
This is very tricky ground because of the practical issue of the baby. If the baby wasn't there I'd say confess. The husband is innocent and absolutely does not deserve what has happened. Still, he is more capable of surviving on his own than the baby. At its bluntest, a parent's needs have to be subsumed to a new baby's for a while.
But in the end - and this is very murky water, there are no good choices here, it's a question of what's the least awful choice - I think that in these particular circumstances I'd cautiously say put it behind her, if she can. She regrets what she did intensely. She was very lonely and under the strain of being a carer and coming to terms with a forthcoming baby. Some people find that a real challenge. Now that doesn't excuse it, but anyone who has been utterly lonely can perhaps sympathise with how overwhelming it can be if the appearance of love, attention, escape is suddenly offered. Especially if it's by a conman who knows just the right things to say and the right buttons to press. (he's the one who needs the vitriol!).
It doesn't make it right, but you could see why she succumbed. She regrets it very deeply. She really doesn't sound like she'd make the same awful mistake twice. Because of this, in this particular situation I think that it has to be weighed very carefully in the balance, to confess or not confess. On balance, given the intensity of her regret I'd say put it behind her. It's not fair on the husband no. All I can say is that sometimes things aren't fair and can't be made right. It's not ok, not at all, but it is the way things are sometimes. It isn't right and can't be made right but perhaps over time it could be left behind.
If she did decide to confess, timing is the important thing - yes, some months in the future when she is physically recovered and can cope with the fallout. that would be for her to know when is best - 8 months, 14 months, it would depend on her circumstances. Before the baby is old enough to remember, certainly.
Regarding staying together for the kids, yes, people do say don't stay together for the kids. One thing that's come out of Mumsnet for me is how often slightly older women say "my parents stayed together for us kids and I wish they hadn't". If the relationship was wrong in other ways, I'd agree. But if it's generally working well other than this awful blip, it would seem to have a chance to be ok. ... if the OP can really put it behind her. As said, I do absolutely agree that it's not fair on the partner but I think that would have to be lived with.