Ah, I'm sorry I didn't read your original post entirely correctly - if your partner started the wedding quest previously but never went through with it, she will have to abandon the previous one first before you could start the new wedding. To do this, she just has to open her Journal, select the quest 'The Ties that Bind' (which would be listed under 'Current' quests), and select 'Abandon'. This should then automatically dispose of her promise bracelet as well while still retaining the wedding dress (if it doesn't, just select the item in the inventory manually and select 'Discard').

This was a point I missed in your original post - your partner would still have the quest active and thus would need to abandon it first before she could go ahead with the new one. It should be easy enough to do so though so don't worry.