OP: Would stitching them back be an option?

Sewing kits are available at many supermarkets as well as John Lewis etc. Or she might even be able to borrow one off someone. I think some women would carry it around with them. I for one do, since I had a button failure at work. It was summer at nearly 40 degrees, so I did not have a jacket to cover it. It was in a rather unfortunate location, confining me to my office until someone found me a safety pin from the first aid kit.

If the safety pins are quite small, they probably won't be detected.
If they do get detected, if they are small, and located where I think they are, they may do a pat-down (or wanding of the area - I can't remember which they do at LHR - it's been a while since I set off the metal detector there) but I'm quite sure they'll understand when your other half tell them what they are for, if they do figure it out.