Wow, tough one. Grandma isn't really in a position to tell the mom how to raise the child. And it won't work anyway. It will only cause tension. I had a meddling MIL that overstepped her boundaries regularly--and not only over religion. Over everything, and it made things incredibly tense.
The child does not need to be told that JW's are wrong. The child needs critical thinking skills. Somebody once told me that they learned how to pick out counterfeit money not by studying counterfeit money---but by studying the real thing. The get to know what the real thing looks like so well that they are able to recognize a fake, even if they have never seen that version of the fake.
So rather than focus on the JW's (because there a Mormons, and Christ Sceint, Fundies, Con men etc.) focus on teaching the child critical thinking so that they can smell a rat no matter what the source. And it would also reduce tension, because grandma would be teaching skills without ever mentioning JW's. Mom may even agree that there is nothing wrong with brushing up on critical thinking, because JW's really believe they are critical thinkers. That through their power of reason they have found "The Truth" and can clearly see what the world can't see. They don't recognize how they have been manipulated and believe they have come to their conclusions themselves.
I know there is at least one book (sorry--I didn't google yet) that teaches you how to teach a child critical thinking. So focus on that----not the particulars of the religion. Give him the skills to HONESTLY come to his own conclusions through his own abilities.
That's what I say, anyway. And oh yeah, if she can talk to mom before the big dip, that would be great too. But I remember that by the time my child was off talking about 'hova and seeing things as good and bad through JW eyes, I was already too far gone. Any resistance would have just grounded me further.