p.s. The logs show
the messages as coming from her subscribed address, but in my
inbox, they look like they're from the non-subscribed address and
it's impossible to glean any information about the group member's
identity.
Your email interface may be showing you just the Display Name
part of the From address. See if there's a way to have it show you
the rest of the address. I have that problem looking at some
member's messages in Gmail's web interface, I have to click the
little "Show details" icon to see the full From address.