That's how gmail in particular and IMAP in general work. Most email clients on mobile devices are smart and don't actually download everything and thereby fill up the memory and storage. But the design intention is that you have access to your complete collection of emails, even if they have to be downloaded on demand by being accessed in a client.

Your friend should look into the All Mail folder on the gmail website to see if the messages are hiding in there.

Hi, thanks for the reply. There aren't any emails in the All Mail folder either :(

It sounds like you created an imap mail account, which shows a remote view of folders on the server. So anything you do either via an imap mail account or via webmail using a browser, you are accessing the same folders. The subscribed folders are synchronised. Any moving or deleting etc done via thunderbird, will update the folders shown in webmail and anything done in webmail will also show in Thunderbird.

Gmail keep all emails in the 'All Mail' folder and by applying labels can make emails appear in other 'folders'. Deleting a label does not necessarilly mean it is deleted from 'All Mail'. If you delete an email and it is moved to gmails trash/bin. It is usually only there for short while because gmail will auto empty the TRash/Bin periodically. When this occurs the email is then removed from 'All Mail'.

https://support.google.com/mail/answer/7401?co=GENIE.Platform%3DDesktop&hl=en

TRy info at these links: https://support.google.com/mail/answer/7015314?visit_id=0-636536913697319100-2121353174&rd=1

This is a contact gmail link to try and ask gmail to aid recovery: https://support.google.com/mail/contact/missingemails?rd%3D1