It's perfectly fine to ask, especially if you explain your reasoning behind asking. You want to create a day that is special, unique, wonderful and totally your own. To do that, you need to get a few of the details about your fiance's first wedding.
If your fiance is ready to move on (considering the ring on your finger, he is), he's also ready for this conversation. So be honest, be open and take notes on what specifically to avoid at your wedding.
Best of luck, and save your friend the Mystery Man some cake.