Sometimes you may run into issues with Gmail that stop emails from being sent or received by recipients.
If you have already checked the JUNK FOLDER and made sure that the mailbox is not full then you can download a delivery report to see what the issue is.
In order to do this just:
1. Go to Google Admin
2. Click reports
3. Scroll down to email logs
4. Type in the user's email address in the from box and hit search
From here you will be able to see what emails he’s getting and if they are being bounced