Here is a list of scam emails similar to hers that can show her how widespread this is LINK

As you have noticed...the email was not addressed to anybody in paticular.This is because it was sent to thousands of people.Your Mother in law is not alone in this "plea for help"
Also,the "lawyers' address is yahoo.com
NO reputable law firm would use a free throwaway email address.Instead they would use something like Lawyername@lawyerswebsite.com

There is no inheritance,there is no lawyer,there is just a scammer posing as a lawyer trying to get people to pay for administration to move large amounts of money
The scammer will have an answer to every question before you ask it so do not try and play him at his own game.
If she was to open up another free email address and mail the scammer under a different name...he would send her the same responses

I hope this helps persuade her...just shout if you need anything else