We need to know the complete specs of his computer including power supply make and model. If it's a newer computer and not too low end the power supply will be the main limiting factor.
If you don't know and don't want to ask him then if you can take the side off the case there is a sticker on the power supply. Copying that info and posting it here will let us know what we are working with.
We also need a budget. You can get graphics cards from $10 to $1000.
The computer model may not help much since Cyberpower computers are custom configured.