Hint: What type of heat transfer is occurring in this situation?
Your friend is coming over soon and you made them coffee. You want this coffee to be as hot as possible by the time they get to you. You can use creamer to affect the coffee, what should you do to make the coffee as hot as possible when they arrive?
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)