[Custer]

Why don't you take her to a nice but not over-the-top restaurant and ask her what she wants. You said she's been expecting it so much that you're very hesitant to put it off, so you wouldn't have been surprising her anyway.

This gives her 2 options. You proposing right then and there without the ring and she being engaged for seeing her "friend" at the wedding, or her knowing that she's about to be proposed to herself when she sees her "friend" at the wedding and therefore not being as annoyed.

The best part about asking her is that its her choice, which will make the disadvantages of either plan tougher to blame on you.