How to enable the 'From:' field in Mail?

FWIW, the "From" field should show by default as with any email client. If your friend's doesn't see it, there's a chance here user account or Mail install is corrupt. She could try creating a new user account and see if the "From" account is visible while logged into the new user account.

What I actually meant to post...

By default, a High Sierra 10.13.6 (17G65) Apple Mail client will show a From: header field in the compose window, as that is what I see here on mine. This is true in Classic mode too. There is no Preference setting, or alternative compose header field selection that provides it.

Is your friend using Apple Mail, but with a different default Mail reader configured in Mail Preferences : General?

Apple Mail does not provide a configurable From: field in its compose panel. When you click on the To: field and type a few characters, Mail will present a sheet containing a list of selectable, addresses taken from the Contacts application, or previously sent email addresses — that match what you have typed thus far.

This is a screenshot taken from my mail app just now (Running El Cap 10.11) - I believe the marked out section is the 'From:' field John is referring to. I also have multiple emails set up so I use this field to select which email to reply from.