I find that in cases like this, when you want someone to just "get it" without having to tell them exactly what they are doing wrong and how to fix it, handing them a bag filled with broken glass and a dead chipmunk sends the right message.
But only if said chipmunk was lured up your arm with bread niblets and taken from its mother. You have to get the full effect going on for this type of message.
I have the answer. Every time she starts going on about her wedding, start saying "oh I know" and then go into a 15 minute long soliloquy about your own wedding. Out insufferable her! Based on all evidence I think you can do it!
"Why would you ruin perfectly good peanuts by adding candy corn? That's like saying hey, I have these awesome nachos, guess I better add some dryer lint." - Nonny
I get the feeling you all think I'm being ridiculous or something. Point taken.
I don't think you're being ridiculous, not really. Your follow up about her actually crying when you said you had other things going on other than her wedding made me wonder if she's really young? Or anxious? Or doesn't have many friends and so has latched onto you?
I do think you just need to smile, say (breezily) something like "wow, you must be getting excited! Can't wait for big day." and change the subject. Not necessarily to the subject of your kid.
If she's friendless, maybe you could plan a shopping/planning day for a few months out, and when she brings up the wedding again you can just be like all "Oh, can't wait to see all your plans and go shopping on August 16th! Now, read any good books lately?".