I find it a bit bizarre that she couldn't manage what you left.
On the other hand, if it had been my Dad left in charge he wouldn't have had a clue! And no one (talking society in general here, not mumsnet!) bats an eyelid at a 70 year old man not being able to cook but expect women to be able to!
So, I suppose it comes down to the fact that your mum (and mil) by the sounds of it, is not a confident cook (or just hates cooking.) Actually, come to think of it, my mum would probably have arrived with the ingredients for a roast chicken dinner in her bag, mil, on the other hand, can cook but doesn't like doing it in someone elses kitchen, and would probably have got quite stressed, although done her best not to show it - which I sympathise with, so maybe it's that.
As this was a one off, I think just let it go, accept she's not going to cook and leave super easy stuff - as it's not a regular thing, I'd leave beans on toast or similar for a quiet life!