How to RollBack changes to Bound WinForms DataGridView row made in another (details) form when the user presses Cancel button?

I have a Form1 (PatientsMasterForm) which has a DataGridView (patientsDGV) in it.
patientsDGV.DataSource = patientsBindingSource;

patientsBindingSource.DataSource = List<Patient> patients;

(patients list is returned by a Dapper as…