If he has a NESPI case +, the power is already directly supplied to the PI3 socket. They were learning from the first version that I have. So no need to replace the cables. Now problem is always the power supply, even with 3A. The difference is that the yellow warning is not based on current, but on voltage. So some power supply just drop to much the voltage. I never had such problem with an Aukru or a NorthPada power supply.
Other story, the additional hubs of the Nespi case with or without +. It is a hub from hub. As the PI3 has a chip with just one USB2.0, multiplied 4x as hub. Now, the Retroflag brothers made from one of this USB hub, 4 new hubs. Not very healthy. So apart some very low power (keyboard, mouse, dolphinbar) you can connect nothing to this USB sockets if you dont want to run into problems. I connected my data stick and my controller emitter directly on the Raspberry USB ports. As my emitter is also a long stick, in this case over a 180° USB angle, and everything is in the box. Under no conditions an hdd will work on this ports, apart if you power the hdd separately.