Hi doggie, I'd say that it's better his fiance knows now the kind of guy he rather than further down the line when there might be a lot more emotional investment there. Just as UM said she's probably being saved a lot of pain by finding out now.
And as for trusting guys again, maybe she's going to be a lot more discerning but that in itself reduces the chances of her getting hurt by guys like your ex, right?? But either way it is probably best that she knows exactly who she's been trusting (your ex), right?
And OK you can take just a little responsibility (if you have too!! ) but he was the one who continued it remember!!
If he felt that he was in a completely committed relationship he could have cut it off at the first questionable message/should have cut it off at the first questionable message.
And you know what, if you hadn't sent those texts then we know that there would have been quite a chance it someone else would have found their way into the relationship, yes??
Now maybe focus as much on not repeating that again on not ending up hurt yourself as well. Then maybe both of you (you and his fiance) are going to be less likely to get "sucked in" by someone like your ex. And then we have: Result!!
Alison