Don't get down on yourself if you had no way of knowing. I don't remember where I saw it but once saw a statistic that 95% of sexual offenders have no prior criminal record before their first offense.
On top of that, I worked for close to four years screening host families for exchange students nationwide, and the majority of incidents happened with very respectable people where the reaction was always "Oh I just can't believe he [always a he] would do something like that!"
All you can do is support the people around you and hope for the best. Seems as though there were no warning signs and there's nothing you could have done differently. You've had your trust in this person broken and that sucks, and you should feel angry and disappointed, but please don't put any sort of blame or anything you did in good faith on your shoulders because then you're not being fair to yourself.
Also I agree with PC, be open and honest with your wife, always but definitely in this situation. The last thing she needs is to find out elsewhere and think you had anything to do with it because you didn't come clean immediately.