I did the "add account" wizard under Settings, and when I go under Contacts->Hamburger menu->Contacts to display, a "Yahoo Contacts" option shows up, but none of her contacts are there.

The People application syncs your Android contacts with Gmail and any other accounts linked to your device, such as Microsoft Exchange or Yahoo. You can add, edit and delete contacts with this ...

And somehow, the official Yahoo Mail app for Android has neglected to sync contact information with the operating system before now.

In addition to releasing a new iOS update, Yahoo today has also released a new version of Yahoo Mail for Android. The update greatly improves the ability to keep track of all of your contacts ...

You can manually add new contacts to Yahoo Mail by using the "Add a new contact" feature in the Contacts tab or hovering over the recipient's name.