If you're lucky it might be just that easy. When you check the breaker and the wiring is at least 12 guage then you might just be able to upgrade the breaker. If not then either install a new outlet or move the toaster to a different circuit. I'm guessing that there's more then just a microwave and toaster on the same circuit. Perhaps the circuit is always providing power to another area of the kitchen or another room. Ask your client if it just started happening. Also, if they just bought a new appliance because it might have a higher wattage rating then the old one. Is the breaker at the panel tripping or is it a ground fault interupt circuit tripping.
You can easily figure out how much a circuit load is needed by figuring how much wattage the appliances or items need then divide that by the voltage(in this case it's 120v) and that will give you the amps needed to run those items at the same time.