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.
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Answer is good but I just wanted to add something. THere is a fairly good chance he'll say something like "i don't remember" or I don't care about the details, I'm bored, etc. You might ask him if its OK with him to get those details from his mother/sister/other female friend or family member that was at the original wedding. I'm sure they could give you a run down of the colors, flowers, food, etc.
Actually probably better that way anyway.
But let him know you're doing it and why lol.