I'm going to date a friend's ex
Why are you going to date that person?
The girl was a nice person and I'd like to get to know a friend a little more
Your friend might be very hurt by you doing this. He might feel betrayed not stay friends with you.
It's not like that, this is different.
What makes you say "this is different"? Did you discuss this with him? If you really think his ex could be the love of your life, you should ask him how he would feel about you dating her, but be prepared to have to make a difficult decision.