Sorry to hear that this happened to your MIL!
It's easy to think & wonder how could anyone get sucked into such a scam but when one is thrown off guard I can understand how it cold happen. Particularly to the elderly who are more vulnerable.
My mother who is suspicious by nature received a similar call several years ago. The caller claimed he was her grandson and even knew the correct name (my son's name) and said he was in a foreign country and needed funds for some reason (which I can't remember). Since my mother is suspicious she asked him a few questions and the call ended with him hanging up. She told me that he sounded different.
Just yesterday I received two scam type of calls...one was a recording from the IRS saying they were suing me & I should call this number, blah, blah, blah...but it was only a recording and I couldn't actually speak to the caller to let them know that I was an attorney (I'm not) and was going to send the IRS after them.
I also got a call from "Ontario". I had Googled the number ...you can do this to see if others have gotten calls from the same number...and it turned out it was Microsoft scammers. So the second time they called I answered the phone with "Hello Microsoft?...are you coming over to help me with my computer problems?"...they hung up!
I can't stand all of these annoying calls! I've been getting more lately and will have to look into getting our number off the list again if it's possible.