Hey Andrea,
Glad to see you are playing around with customization. If a user makes a change in QB, we cannot make an action run in Method just by the sync. You would need to go back into Method and click a button to start the action sequence.
A thought that I had was;
(all of this is done on the customer list screen)
1. Create a new text field that will also store the users email address. (2nd email field)
2. Edit the select button in the existing users grid.
3. After the action, "set active record ID" add actions to check and see If the 2nd email field is blank.
A. If blank, copy the original email address and place it into the 2nd email field.
B. If the field isn't blank, compare the original email to the 2nd email field, if they are different, send an email.
C. If they are the same, do nothing.
4. Copy the new email address from the original email field, place this new email address into the 2nd email field that you created.
The idea here is that the 2nd field will not update if you make a change in QB, but the original field will. So whenever you select a customer, it will check to see if the email was changed by comparing the 2 fields.
You will need to update you "on text change" so that if a user does change it in Method, then the 2nd email field (the custom one you created) gets updated with the new email address.
Hope this helps!