I think you are right that it would have happened sooner or later. People don't text like that and are then have the physical opportunity and privacy and don't do anything.
The only way this wouldn't have gone further is if one or other of them had baulked at the reality of it, when it came to it - but it would have come to it. And at that moment they would either have gone with it, or one would have backed out.
The point is, you know your husband would have got to that point, and you then don't now whether he would have backed out, or gone ahead. There is more chance he would have gone ahead.
That's all you know.
Anything he or she tell you can't be taken as truth. They might be telling the truth, but you don't know. What you do know is that you can trust him and her to lie, and to hide things from you - because this is precisely what they had been doing, with their texting at least.
You cannot know at all whether they had done something or not already. I know the texts sound like they were on the brink of it. But you still can't quite know what had gone on.
And in one way, it doesn't really matter whether they had actually touched each other or not - they had confessed their attraction to each other, and they were in the process of delighting in the seduction and the foreplay, well on their way to the moment of consummation.
They had already agreed to go ahead, if not consciously or explicitly. Although from what you say, they were pretty explicit.
Their reactions were typical - I heard the same, although the texts I saw were less definitive - that this was just a flirty game, that they were just joking, that they were just great friends, that it meant nothing, that nothing had happened and nothing would ever have happened. Almost verbatim to what you describe.
But, I'm afraid, all the while they were just regrouping and working out how to proceed more privately and more securely. They had no intention whatsoever of stopping.
Which is why I say that the very fact that he had got this far means that he would have gone further, and you can't be sure he would have stopped.
Some people do stop. Some people are doing it for the wrong reasons, and this becomes clear to them, and they stop. They are repulsed by what they had been about to do, and they don't go ahead. But that is really only mostly if they were doing it out of some desperation or madness in the first place.
Those who simple stumble across someone and fancy them, and find that feeling reciprocated, those people - I think they are the ones who go ahead. And perhaps it is short-lived and they come back to reality one day. Or it is too strong a connection, and they carry on, and devastate built lives around them.
You asked if you could ever trust him again. I think you can't trust anything now that ever comes out of his mouth. And I say that in a neutral way. He has simply proven himself capable of lying, and willing to do so. And now you will have a much harder job of knowing whether he's telling the truth or not.
Have to go now but more of which later.