Make the non System Admin a Site Administrator of the site
The non System Admin should then enable password protection on the site, adding a secret password. The password should not be shared with any Collaborate System Admin. (The non System Admin may share the password with another person on their team, if they like, so the password is not accidentally lost.).
In the presence of the non System Admin (or another user who knows the password), a System Admin should then attempt to access the site, but permit the non System Admin to enter the site password manually on behalf of the System Admin.]
The System Admin will then set the site status to Read Only and then log out of Collaborate