The advice you got so far is right on target, but I couldn't resist adding my own as well.
My wife didn't want a diamond. For many reasons. So when we got engaged, there was no ring present and then later she kept an eye out for the kind of ring she would want. When she saw one, she showed it to me and I got it for her.
Bought it at "The Museum Store" in the mall. It was a reproduction of a gold wedding band that was supposedly popular during the Renaissance. It's a band with a pair of clasped hands. I've seen other similar ones called "Fede rings." Hers looks just like
this one, but in gold. Added bonus was that the money I could have spent on a diamond ring was spent on a much nicer honeymoon than we would have gone on otherwise.
Anyway, let her choose the ring. And leave your buddy at home. This is a time for just you and your future wife.