Speaking from my experience in Tech Support for Microsoft, in situations like you describe it is almost impossible to speculate on what happened unless you can directly observe the process while it is in progress. All I can offer is a process that mitigates the risk of loss. Assuming your wife was using the Apple Mail app rather than deleting the portion of the message she didn't want it is safer — and IMO easier — to…
  1. select the portion of the original you want in the reply or forwarded message
  2. then click on the appropriate send or reply icon
  3. add her text, forwarding addresses, etc. in the draft window that appears.
  4. When editing is completed click on Send

If there is an error the original message has not been touched, so it is easy to start over from scratch.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein