Blaming this on intoxication on the part of the OP's mother-in-law is incredibly offensive. Did you read the post? The FAs came to rush the F pax off, after iniitally being told they'd have to wait a long time. so everyone rapidly gets their belongings together in a few seconds and rushes off, as instructed. In such circumstances anyone could forget about a coat that had been taken from them many hours before, especially with so many other things that tend to be running through the heads of those that arrive in foreign cities. It's possible she'd had a drink or two but unlikely - alcohol is not routinely served with breakfast prior to landing, even in First.

It was an accident. Both the FA and the passenger can hardly be said to have demonstrated exceptional stupidity, intoxication, or lunacy. The same sadly can't be said of BA's procedures for handling luggage left in the nose of planes.