I don’t think I really solved it, but it’s been a while and my memory could be failing me.
I tried to add the secondary email address as explained here:
but that didn’t work because the email addresses were taken.
I think the solution would have been to merge the users, though I never proceeded with it because I just talked to the user and she has been using only one of the accounts since, so I can’t tell you how that would have worked in practice, sorry.