Ok, read this again and now I'm assuming Outlook is missing gmail email.
So many things, first I'd ask her if she tried to fix this herself, or a family member or yet another tech before you. If so, then stop what you're doing and go into backup mode. I see this all the time in my field, people have mucked with the Outlook bad before calling me and the data is there usually somewhere else, wherever the other person lost it. IMAP for Outlook can break and email will be in Outlook that's not on the server and vice versa. POP emails can be in another computer or another profile with a PST.
Too many variables here. If you're stuck this week, I'm happy to help you at no charge to see what I can find.