Acess via Mail to more devices

I have a MyCloud Mirror, a buddy has exactly the same device. He is registered on his device with his e-mail address in order to be able to access it externally.

Now I have set up a user share on my cloud and of course also used his mail address. Since then he only comes to my cloud and not his. Since I deleted his email address in my cloud, he also has access to his cloud again.

How is it possible that he can also access my cloud? Does he need a second e-mail address or is there another possibility?

I’m grateful for any help!

Thank you