I have all kinds of thoughts on this.
Why the heck would your wife go out "socially" by herself with this guy, and you and her not think it was going to lead to sex, especially since you already knew this guy's intent was to play with your wife, AND you and your wife already decided it was okay if she played with him?
I think your wife had sex with him. That's my take, and she's not willing to admit it (for some reason) but wants to fuck him again.
Did you have some rule against them playing? If you did you didn't mention it in your OP, I got the opposite impression...you agreed to bring in a solo playmate for your wife.
And this "taking it slow" thing, what did that mean to you, what did it mean to your wife?
Also, did you talk to his wife about her husband playing solo with your wife? I ask, because it didn't sound like it. If this guy e-mailed asking to play with your wife when he travels through, it may be without his wife's knowledge and approval.
You didn't say why your wife's last relationship with the solo play partner ended badly, but that may play a role in this story. Maybe you can tell us more about that.
From what you've provided thus far, I think you and your wife need to talk a whole lot more without blaming each other for a problem that has arisen because both of you failed to communicate clearly from the start.
LM