J,

Oh, you mean the display name of the email address, not her groups.io display name. But that's not it, either. It's showing "xyz@....

Correct, I was thinking that the From field might say something like:
From: "xyz@..." <subscription-address@...>
However...

 I asked the member in question about it this morning and she said "oh, I sometimes post from my other email address, sorry." But I don't know how she can do that without having an alias.

That does sound like an Alias, except that I thought when I tested it that the alias in a Groups.io account put the subscription address in the outbound From field, so you wouldn't see this confusion.

Possibly the alias is staring me in the face on her member page but I'm still not seeing any. I've emailed the details to Mark at support.

I'm interested in what you find out.

Shal