Hint: What type of heat transfer is occurring in this situation?

The primary mode of heat transfer before the creamer is poured into the coffee is radiation. When the creamer is poured into the coffee, the primary mode of heat transfer becomes convection.

Since convection is a faster mode of heat transfer, then, assuming that the creamer is colder than the coffee, you’d want to minimize the amount of time spent with convection being the main mode of heat transfer. As such, you should pour the creamer into the coffee right before your friend arrives (or even better, serve it to them without creamer).

There is still convection while the cup is sitting on the kitchen counter. I’d add to your solution and say that its better still if that convection is interrupted (place a plate over the cup)

Here’s a different take. When the coffee is freshly hot, the temperature difference between the room and the coffee is at its largest and therefore temperature drops the fastest, initially. As time goes on, the rate of temperature drop decreases as the coffee approaches the room temperature (think lumped capacitance model equation for temperature). If you pour the creamer in immediately, the temperature will drop quickly from the creamer, but then there will be a smaller temperature difference between the coffee and the room, therefore reducing the rate of heat transfer. If you wait for your friend to come and then pour the creamer, then the coffee would cool quickly from the large temperature difference and then drop more as you add the creamer, possibly more than if you had added the creamer immediately. Depending on how long your friend will take, there is probably a cross over point for which method is better. For a short wait, wait to add creamer. For a long wait, pour creamer immediately.