I think I'm going to date the girl that my best friend chose to stop seeing after a few dates.
Why would you date someone your friend used to date?
I like her and I don't really care that my friend dated her a few times.
If you and the girl and your friend are all fine with your relationship, then you don't need to worry about anyone else's opinions. But if it causes tensions in your relationships then you might have to choose between your friend and your girlfriend.
You're right, I'm going to date her because I like her.
I think it's good that you've found someone that you like and as long as everyone is honest, I'm sure it will work out well for all of you.