Maybe you might question some of your basic premises, which I think are questionable. If not, I think you are on borrowed time with this friendship. I say that because I think you are avoiding the elephant in the room, which seems to be under-addressed in this thread. And I think he's mindful of it:

So you are pleased that he will make sure that his relations with this lady "are as pure as possible," so that there will be no questions coming from you. So that resolves things, kinda. If this guy has even the flimsiest of backbones and even the weakest of personal boundaries, he's going to wake up one morning and think: "Why do I have to make utterly certain that nothing in my behavior should, in the least way, raise up any suspicions in her. Why does she get to call me on the carpet? Why should I allow that?" It's nice that you admire his character, but doesn't he get to decide what his character can be? He is gratified that you approve of him. But maybe he'll decide: "I'll be who I choose to be, and she can take it or leave it."

Why does he have to live up to your standards of who he should be? Because you are his friend gives you that right? I don't think so. And I'll bet he doesn't think so. Maybe I'm all wrong. You can disregard me totally. But ask yourself: What do you think is still bothering him, despite all the making up you did? You're claiming authority over him that you'll have a hard time asserting over your own children, whom you do, or will, have a duty to set standards of behavior for.