Best Female Friend Married..How To Approach Husband?

Have an unusual situation here...at least for this group anyway.

I've been married almost 15 years...3 kids. Other than my wife, my best friend is a woman who I've known for many years. We are brother and sister practically. My wife has been supportive and my best friend also thinks highly of my wife and they have hung out on their own, as well as part of a group.

My friend married her boyfriend of several years finally...about time, lol. He's a good guy, and I'm very happy for them. The only times we've ever talked is when the four of us happen to be together or I was with the two of them. He does talk quite a bit when given the opportunity to open up.

I want to know how do I approach him and let him know that I want to consider him a friend also? It's a subject I broached with his wife a couple of times in the past, she said it was up to him for the most part. I also realize that now things definitely change with them tying the knot. Much like when I got married, my wife and I are a set and she comes with me. They are now a set, I don't get her without him. I want him to know that he is welcome to attend events that I invite my friend too (she's been active in my kids' lives as well, with birthdays and Christmas, etc). Perhaps it would be possible that eventually we can become friends on our own terms.

I come asking for sound (or somewhat sound) advice. What say you, fine folks of the Marriage forum?