How can I: not update key (ref) when someone else edits record?

[dw2020]

I currenlty have records for staff, which they can add their notes to. Their manager can update these notes but when they do, it updates the key/ref (email) to their email. How can I stop this from happening but still enable a manager to edit their own team-members’ notes?
I’ve tried a few different ways eg adding an edit_if constraint but I’m not able to add that to a ref-field.
Thanks in advance.