If I make an admin account on my Mac (running macOS Sierra) for my friend, will they be able to see the contents of my account (also admin) (Documents, Photos, Trash, etc.) or are they walled off between admin-to-admin accounts?
Without further changes an admin can't read the content of the default folders of another admin - except the Public folder. Any custom file or folder (e.g. a dev folder in the user folder) can be read though - provided the default umask (022) hasn't been modified previously.
Any admin can either reset the password of another admin or change the permissions of the other admin's folders with
sudo chmod ... though and he/she will be able to access those "protected" files and folders then.
Adding an additional admin makes your files vulnerable to him/her eventually.