Transfer from one NAS to another remote NAS

I have recently purchased a My Cloud EX2 Ultra. My friend also has the same NAS. Is there any way of him transferring files he has on his remote NAS (on a different network to mine) directly to my own NAS?

I tried creating a share and giving him access as a user, but there doesn’t no seem the functionality for him to copy or move his files into the share. He can only view the folders of the share.

Idea would be for us to have a share on each others drives and the functionality to share files remotely to the others NAS when required.

Any ideas?