A 3500 will likely work, but at 15A, won't give much leeway in the kitchen, especially if he wants to make a pot of coffee or use the microwave without unplugging his fridge and freezer. If he can afford it (seems like a small difference at this point) 5000W would be safer, or even 7000W. A lot depends on which phase everything is on.
Anecdotally, I can easily power every receptacle and lighting in my house with my cheap 3500, with all TVs, fridges, etc, on... and ironically, is too underloaded and puts out too high a voltage until we turn all the lights in the house on to inrease the load and get the droop down. (My computer UPS beeps incessantly about it!) It's not enough for the heat pumps, range or water heater, though.
