Those steps will work. You could also replace the first two steps with this string:

older:2017/04/01 is:unread

You could also eliminate the largest emails first, by searching for everything over a certain size, by adding the size flag:

older:2017/04/01 is:unread size:5m

That will search for everything unread, older than April 1st, and 5MB or larger.

But the first thing I'd do is delete the stuff in her social and promotions categories to see how much space that saves you:

older:2017/04/01 is:unread category:social

You might be surprised how much space freed up by that purge alone.

In the case of all of these searches, you'll need to do what Archeon said and select all the conversations, then click on that link at the top of the page, then delete.

To then free up the space immediately, you'll have to go into the trash and empty it. Otherwise you'll have to wait 30 days to see the space free up.