It is not possible to restore deleted attachment from email, but, maybe, it is still not lost forever. If user leaved this attachment open when Outlook is closed and not a lot of time pass after this action, then in can be found in the internet temporary files.
For Office 365 the path to this folder can be:
%localappdata%\Temporary Internet Files\Content.Outlook
Under this folder can be found folders with cached files. If file exist, it should be in there.
Path can be also found in registry Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Security as value of the record OutlookSecureTempFolder.