How to accommodate changed yahoo mail password on iPad?
As @Tyson mentioned, your friend will obviously need to reset her Yahoo password through Yahoo directly. Once that's done:

If she's pulling her email in the Apple Mail app, she can update the password in Settings (the gear-like icon) > Mail, Contacts, Calendars. Once she navigates there, she needs to select Yahoo under the account list, then select the top "Account - (email address) >" option which will then give her the opportunity to enter her new password.

If she's using Yahoo's own "Yahoo Mail" iOS app, I'd imagine that the settings are within the app itself (sorry, I don't use Yahoo Mail anymore).