What a minefield!
I think you have to act with extreme caution. But, speaking as one who got drawn in to the cult and have only comparatively recently got out, I think you should err on the side of protecting your wife from them.
Five minutes on this forum are enough to persuade anyone that this is a very dangerous high control cult and that any "friend" communicating with your wife is a coompletely brainwshed automaton, controlled by the Borg.
When a parent dies, families and individuals are at their most vulnerable. it's a time when relationships fracture. I have seen this in my own family, and a wise friend said the same to me at the time.
The very best thing you can do for your wife is to share your feelings for her with her. And you're going to have to play it by ear. There's nothing the matter with a shared email account; I have a number of married friends who do the same. Not sure I'd like it myself, but then I'm a very independent sort of person.
You are absolutely on the horns of a dilemma. Yesterday, I experienced contact with someone who was a dear friend within the JW's who contacted me and I tried to keep the conversation away from the WT...and she just couldn't. it didn't shake my resolve but it certainly did get under my skin in a very subtle way, so....don't underestimate their power.
On balance, I'd do two things:
1. Talk to her carefully and express your concern.
2. fill your lives as much as possible with things that have nothing to do with the KH in any way, non-JW family, friends, other things.
Is your wife clearing out her mother's effects? That makes her even more vulnerable. It's very hard. I know. I've been there. Emotions run very very deep.
Please let us know how you get on. Thinking of you.