If the other facility is subject to a different FEM assessment, you can note in your assessment that you're purchasing electricity from a third party. If you have a specific agreement in place for the purchase of electricity generated from diesel with that external facility, you can include this information in the 'kWh of purchased electricity' field.
Essentially, if your facility uses a diesel generator for standby electricity, and it is located in a different area not covered within your FEM's scope, you can add this as 'purchased electricity' and specify the emissions factor for it.