How to delete 35K emails on a iPad Mini 3

If you have web access to the mail account, I would log in there and delete the mail. When you go to the account on the iPad, they will have disappeared. If this is not an IMAP account and they are all mail that is stored locally on the iPad, I would delete the account from the iPad and then power cycle the iPad, then add the account back again. The mail will be gone then.

Alright, if she is using it on her iPhone, are the 35K mails on there too? Did you check to see if this is a POP account? My last question would be, since you say the iPad is hers, why are you trying to delete the email? Are you trying to use the iPad yourself for something instead of her? Can you explain the entire situation so I have a better idea of what to recommend?

In the Mail app, tap Edit at the top right, then tap Select All, then delete.

One way would be to remove the email account from the iPad. This would have no effect on her iPhone.