Cote, I must urge you in the strongest terms possible to stop this in its tracks.
I understand very well your belief that stopping this RDV does not address the underlying problem; that he would have been unfaithful unless thwarted, but confronting this situation now does not mean the situation is concluded - far from it.
Your H sounds like the acrchetypal "rescuer" - just as Counting says. He likes to be needed, but of course that also includes succeptibility to flattery, adoration and since she's an ex - lots of reminiscences about a time when he was younger and probably more beautiful.
Unfortunately, men who respond to damsels in distress like this, tend to be the sort who become emotionally involved and cannot separate sex from emotions. Therefore, what might have started out as a bit of an escapist adventure becomes horribly messy and confusing once sex is involved.
There is enough here in the open domain for you to have every right to confront and express your displeasure. As he would, if you were asking to send an ex money and he'd seen messages from that ex about a secret meeting and messages about you getting cold feet.
If I had to guess on this one, I'd say she has been asking to meet for a while now - and he has been half-heartedly refusing her. He probably wants to be wooed and chased, but there's also possibly a large part of him that wants her to take responsibility for his infidelity. It has probably got to the point where he's now telling himself that he's not as much to blame, because she chased him and he resisted for a long time. Now, he's probably going through a series of "why nots?" to justify meeting her next week.
When a man like this is unfaithful for the first time, he feels like something has been lost that can never be replaced. It is a horrible feeling, even if it is entirely self-inflicted. To make him feel better about himself, he usually gets briefly more involved emotionally with the OW, if only because he needs someone to tell him that he is not too awful after all - and the deepening feelings serve to mask the guilt he feels.
Now, you're not daft enough to believe that a secret meeting during a non-existent business trip could ever be innocent; no secret correspondence is ever innocent, after all.
And if you stop this happening now, don't for a minute accept any nonsense about how he was never actually going to go through with it, because that would be a lie. What you will have to do though is get to the bottom of why this has happened - and what it is about him and his personality that likes to be wooed, chased and to keep secrets from you.
If you do stop this before it happens, there is a long road ahead to unravel this behaviour, the aim of which will of course be to affair-proof your marriage so that you are both utterly committed to fidelity. The mistake many couples make after an adultery attempt is foiled is to think that this is the end story. It never is - if no work is done by the thwarted adulterer, it will just happen again when another opportunity presents.
But I can tell you that your personal recovery, not to say your marriage's survival, will be much riskier if you let this happen, all so that you can have incontrovertible proof.
If your H is a fundamentally decent man, he will thank his lucky stars that you intervened when you did. Not immediately perhaps, but eventually, absolutely.