Please check to the coinbase support to utilize the trading option least.
S/he would need to first transfer fiat and exchange it for some cryptocurrency, and then convert that to BTC or any top coin of their choice, then transfer that to coinbase
You are funny, your explanation may not help him. Even I know to say the same thing what said here. That is the way you do that. You need to explain with full step by step or ask the person to write or check their information board.
Hope you will not repeat such explanation to anyone.