Are they all the same flavour?
Are you sure you are reading the best before date, not the date of manufacture? I know that sounds daft, but somehow the date of manufacture always seems to stand out more and I often mistake the two on a quick read.
It's really weird that these are so out of date as they have a long shelf life, so it would have been in her cupboards for an awful long time for her to keep 'missing' if you know what I mean, and then keep missing them for another 6 months. Very strange.
Not disbelieving you or anything, but that's why I wonder whether it's the date of manufacture.
It's easier to get it wrong with tetras as they go out of date much quicker. Oh and you do sometimes get the odd duff tetra even within date, but it's pretty rare.
So, anyway, if they are out of date, then yes, contact your CDC to get them exchanged. I'm sure she'll be gutted but definitely needs to know!