Thanks to international money laundering and payment scams and such, there’s no good and easy way to do this.
Apple does not allow international payments for online purchases. You’ll need to pay in US local currency or a payment card with a US address in the US online store to purchase an iPhone for somebody located in the US.
Beware here, too: Gift Cards are also country-specific.
Purchasing the iPhone locally and providing the phone to them directly can work, but the iPhone warranty is local to the country of purchase and international only at the discretion of Apple. (Check the Apple Warranty Terms & Conditions for details here too, as this has changed over time.)
Probably the easiest way is a wire transfer into their US financial account, and they then buy it locally in Los Estados Unitos They can also get exactly what they want for capacity and color and options with this path, too.