@scooke Hi, sorry for the delay.
Yeah, @girish was able to solve the problem, which was not a technical one, as usually is the case with these things, heh.
The email she uses is an alias, and not <username>@<mydomain>, and she hadn't used it in a while. We're both pretty sure she used to be able to log in with either username only (no '@domain'), or with her aliased email <alias>@<mydomain>, but it seems we're both wrong on this.
So I tried to log into her email with <username>@<mydomain> and, lo and behold, it worked.
She doesn't use this email at all, so I'm talking to her and seeing if we change her username, since there's only a few apps and email will be the most important one from now on.
Thanks everyone for the help!