Depends on your idea of 'moderation'. I'll have pizza maybe once a month as a treat, and I don't worry I'll keel over and die. Same goes with popcorn, beer, ice cream etc etc. But I don't think you can have 'moderation' with eliminating grains. You can either eliminate them completely, or not. Moderation to me implies your wife is eating popcorn multiple times a week.
Take fruit slices into the cinema with you, dates, jerky, sweet potato chips. Kale chips have all the saltiness and crunch of popcorn that might satisfy your wife. Be creative!