Good for her! Sounds like a lovely event to honor a lovely relationship.
I second the" get a nice new something from Williams Sonoma or Crate and Barrel" idea. Every year I buy a new item for my kitchen to make holiday preparation easier. She must be in need of something.
Or consider very nice picture frame.
If the wedding is this month or next month, you could get a special Christmas ornament. I'm not a trinket person, except when it comes to the tree.
If they have a balcony, consider something for dining outside.
A picnic basket with a bottle of wine and a corkscrew and wine glasses. Toss in a picnic blanket.
Two place settings of beautiful china for dinners for two and a highly rated cookbook you know she doesn't have.
A crystal vase and two dozen long stemmed roses in it.
A pot by Le Cruset (?) --enamel covered cookware. It works like a dream.
Enjoy the ceremony.