I can understand your frustration.
I am still missing information.
Please don't confuse Microsoft Office Outlook and Outlook.com (used to be Hotmail)
You say he has just Windows 10. The operating system has nothing to do with the operating system. Defender has nothing to to with messaging either.
I suggest we concentrate on the Gmail issue. Are you using Gmail directly from the Gmail website? When you press reply, do you get an error message? Are the reply messages in your going to a folder, such as sent?
I strongly suggest that you have your internet connection verified. Any type of disruption in the connection will disrupt Gmail functioning.