You have to get permission. It is the code. You have to ask whether it is ok to date her.
I had a similar situation happen to me several years ago. Long story short, I asked permission, but did not get it, and the girl ended up getting pissed off at me because she did not understand why her ex would be in a position to decide.
I kicked myself in the rear for a long time after that. This girl was not only beautiful, but very refined and someone that I could see spending a long, long time with.
In any event, it was the right thing to do. I was able to live with myself. I ended up meeting my wife, and we have two great sons now.
If you do not ask permission, be prepared to lose a friend and, perhaps, some of your mutual friends. That happened with a guy I knew. People get pissed, end up hating each other, and then the mutual friends are put in a position of taking sides.