To clarify (I agree this was poorly explained, although perhaps could have been deduced from the following explanation) I should have said it was unlikely she had a drink after sleeping and prior to landing. Drinks are not routinely served at that time. I don't think that the champagne seven or eight hours earlier is likely to caused her to forget her coat. The general rush and confusion can't have helped, and she might not have been fully awake, but that's hardly her fault.

I think in the circumstances both the mother-in-law and the FA made quite understandable minor mistakes. It's what followed that which is really the issue.