Key Largo: the coral is not "wiped out". The current state of coral reefs worldwide is a serious issue, and the reefs from Florida to Cozumel have been experiencing something unusual recently. But it's not as if all the reefs have disappeared. If you need a low-stress diving environment to allow your wife to develop skills and confidence, I 100% agree with the Key Largo recommendation. Cost, ease of access (why leave the country?), top medical care, plenty of dive ops, shallow reefs, tons of life... it's a no brainer.
Cancun and hand grenades: I just got home from Cozumel late last night. I was there for the past week. I didn't see or hear or hear of any hand grenades. Or any kind of cartel activity. Granted, it's not exactly Cancun... but it's the same neighborhood. Oh, and our flight was full. So if there's a problem with the Cancun/Cozumel area, it seems the majority of people travelling there have no knowledge of it.
Keep in mind that anything you hear about in the popular media will have been "spiced up" to grab your attention.
However... even assuming there are no hand grenades or any cartel activity in Cancun, I still vote Key Largo >> Cancun for what your wife needs.
So, some of you DON'T know - okay my friend - here is a link to an article about just one hand grenade attack this past November in a Cancun bar. (URL if above link doesn't work: GRAPHIC -- Four Injured in Grenade Attack at Cancun Bar)
Here is another link (GRAPHIC -- Cancun Wraps 2018 with At Least 540 Homicides)
I