Recently I received email from a trusted correspondent and carried on a back-and-forth discussion only to discover that the correspondent had originally started the exchange via a Hotmail account (which supposedly is never used).
Clearly, the correspondent needed to be signed in to send and then receive responses to the e-correspondence and should have been able to tell the difference between the Gmail account (and its GUI) normally used and that of Hotmail.
Alternatively, since an exchange between me and the correspondent ostensibly went to the correspondent's Gmail account, something is even fishier, given that the address used was hotmail.com. When I checked under "Show original" the initial message came via hotmail.com and my response went to hotmail.com — all confirmed as legitimate by Gmail.
To my mind the correspondent's various devices (iMac/iPad/iPhone) must have been hacked and hijacked to disguise that.
I suggested that the correspondent (who is otherwise unsavvy in the ways and wiles of malware and such) go to the Apple store to have the problem investigated.
As I see it and have recommended, the correspondent's devices have been compromised, which means that any sensitive information (including financial info, passwords and such) may be open to being stolen and therefore all such info on all devices should be deleted — although it's likely too late to close the door.

What say the assembled gurus on FTM about this issue?

....or, while he might not be technically savvy, he could possibly have both hotmail and gmail accounts being accessed through his Mail app rather than through the web interface that would differentiate them so visibly.

It's quite possible to initiate an email from one account inadvertently, then respond to the reply from the opposite account.....that difference in the Mail app user interface is a bit too subtle for my taste.

I have multiple email accounts going concurrently on my Mail app and have found myself in just the situation you've described.....no outside interference required.

When I passed on your comments qua explanation, it seems that the Mail app on the iPad was indeed the culprit.
Disaster ostensibly averted, and my adrenal glands have reverted to normal.

Deep cleansing breath, calm thoughts of warm puppies, cuddly kittens and long walks at sunset on the beach in the Bahamas.......and just because the hackers weren't involved this time, doesn't mean that the next time will be as benign.