Yeh, he could at least you the check box at the top of the list on a give page to select all items on that page and delete them.
I have a Yahoo Mail account but haven't used it much, and there's a later version which I have not used yet. Anyway, I see a button to view All Messages. I presume that means it does just that, regardless of how many messages are in your Inbox. If so, then cleaning out your Inbox should be easy unless you need to do a lot of screening out of certain messages.