Lets go to the facts first.
Let's call my close friend Bob, and his ex-girlfriend Jane.
Me and Jane go way before I meet Bob. Actually, she presented me to him. Since then, we became really close friends, at the point of starting a business together.
I have known Jane since high school, but never had anything romantically with her. Just friends.
3 or 4 months ago, Jane dumped Bob (they dated for about 8 months), and I was there for him. Stood up for him and went to a lot of places to make him "forget" her. And he did.
Bob knows that we are basically the same, so, the first thing he said to me after the break-up was that "You are free to see Jane if you want to, because you are very much like me."
He probably was sadly mad with her when he said this, and I had no intention whatsoever to go after Jane. But time changes, and after Bob, we started going out, we became closer and now we are going on dates.
I tried to bring up the subject "Jane" to Bob, and he was kinda mad, saying that I would not work with her. (To be clear, I haven't said anything about going on dates with Jane to him).
So, this is a big IF, but a pretty possible one.
If things work it out with me and Jane, I mean, a more serious relationship, I'll have to tell Bob about it. Believe me, having to keep secrecy is not cool.
How should I approach him if it came to be?
After some time together with Jane, I could clarify in my mind that I'd rather have Bob as my friend. So, I choose to stay with him.
Thanks for all the help!