Hi there and welcome
Gmail will not have added it Phil, the system has no way to actually do this.
Ask him to forward the email to you, is he able to do this?
ok Phil, can I ask what more information you need? As its not possible for Gmail to add people to emails, it's just not done.
It has been either added by accident or forwarded from someone else, but to know for sure I would need to see his copy of the email, then I can tell you Phil.
the only thing you can do on your side is to always check he is not in the To address list. It is possible he uses another email address that is forwarded, so it may not be clear if your adding him or not.
The only way for me to confirm is to see the email he received.
ok... please understand Phil I do not have all the information from you I would need for this....
Can you please obtain the email from him, so I can give you a complete answer.
No problem Phil, then I cannot give you an answer you will find acceptable as you feel Google have done this.
I have decided therefore to opt out and stop working with you
thank you and all the best