Well, everyone I made it through the evening, and, honestly, things went a lot better than I had anticipated. But here's a synopsis of how it went (I'll try to make it brief).
We went out for dinner and I didn't want to waste time, and hit him up immediately about what was going on. He seemed to be in a better mood (or, at least, less all-over-the-place about things). He assured me that he was taking matters into his own hands and had done his best to avoid this girl, and lessen their conversations since he had last spoken to me. He obviously knows what the correct move is in the situation, and, he also guaranteed me that he was guilty of nothing more than flirting due to the flattery he was getting.
He kept saying over and over how nothing had ever happened like this to him before, and that when he had spoken to me about it last, it was all new and he was afraid of what he was feeling. He was confused and startled by his own feelings. I asked him about the whole conversation we had about "the one" and he said that it was just all part of the confusion because it was a topic that the girl had brought up during their conversations. But, he said, as he distanced himself from her, that "feeling" or whatever, had faded, and things became more clear to him.
When I asked, he told me that everything was great at home, and that there were no underlying issues with anything, beyond the fact that he's been a little stressed with work and extended family issues (his parents).
Now, I let him tell me all of this, but, as much as I wanted to, I wasn't just going to just take him at his word, without getting my chance to lay everything on the table. I pretty much went through all of the stuff people talked about here, even citing some of the examples some people have given from their own lives (thanks DISers, and I hope you don't mind, you all stayed anonymous). I particularly loved the posts people made about if this girl is looking to cheat with him that she would probably cheat on him, and then what it would have been for? So, I definitely threw that in there as well. I told him about what it would mean to the lives of everyone around him if he were to make such a terrible decision. I also let him know, in no uncertain terms, that it would be a terrible decision, and one that I absolutely cannot endorse.
We finished dinner early, and were supposed to go out to a bar for a few drinks, but I suggested, instead, that we pop back by his place so that he could help put his daughters to bed. (I was proud of this decision on my part, I guess I was feeling empowered thinking I was helping him). I felt that by doing this it wold reinforce what I had been just telling him about his family, etc.
Afterwards we hit the bar and had a lot of fun talking about the normal guy stuff - sports, baseball, etc. It really felt like things were kind of "normal." At the end of the night, though, I made sure to bring things around to the original topic again, and I made my last "tough" stance. I told him that I really hoped he was serious about setting things back into balance and not making a decision to put himself in a bad spot. I wanted to be sure that he knew I was NOT going to put any blame on this girl if he did something, that HE would be 100% to blame in my eyes. I told him that the ball was now in his court to prove to me, and his family, that this was nothing more than a speed bump in life. I also said that I expected this to be the ONLY time we had such an in-depth discussion about this situation, because if it continues, I'm not going to go through this again, because my opinion is NOT going to change, and he would be unable to count on me to assist him.
FWIW, and I know we can't get into heavy religion around here, but I did take some PP's advice and told him that I think he and his family need to start going to their Church again. They used to go every Sunday, but stopped several months ago.
So, that's it. As I said, I thought it went well, and was surprised how easy it actually was to tell him how I felt. Once I got rolling, it really started to get easier. I'd like to chalk it up to everyone here who helped me organize my thoughts during what was an extremely stressful few days approaching this.
As I said, the ball is in his court now, and I hope he was serious about everything, and really takes into consideration all the things I told him. There's really nothing else I can do now. Again, thanks to everyone here who gave me their thoughts.