You can add a client warning that will popup when accessing the client record or making an appointment for that client's pet. You can also add an invoice warning that will popup when creating an invoice for that client.
Open the client card and click on the General Information tab.
Add a warning in each field.
Go to File, Lists, System Options, Local tab to turn the warning popups on/off.
When you open the client card or make an appointment for one of their pets, the client warning will popup on the screen.
The invoice warning will popup when you create an invoice for that client.