Password reset requires 2 security questions (ugh - already ugly) and while she's 100% certain that she knows the answer to both the second one isn't accepted - probably another spelling issue (think St. Marlo vs. St Marlo vs. Saint Marlo vs. Sankt Marlo vs ..).
All of this is her fault, not yahoo's. But now she's stuck. There are no ways to contact support, at all, and by now her 'resolve this problem' links already contain a "In rare cases like these, we suggest creating a new account" line.
Anecdotal moral of the story: Yahoo has no customer support at all. Migrate your elder family members away while you still can. :)