If it's three weeks or less, T-Mobile has a non-renewable tourist plan that costs $30, has 1,000 talk minutes, unlimited text, and 2 GB LTE data (throttled to some low speed, but not shut off, after 2 GB).
If T-Mobile's coverage is not sufficient, AT&T Prepaid is another possibility. Their basic plan is $35, has unlimited talk and text for a month, but only 1 GB LTE data. Walmart and similar usually sells AT&T SIM cards that you can buy and bring to your friend. You can also order a SIM directly from the AT&T web site. https://www.att.com/prepaid/
There are also a number of MVNOs that might be a little cheaper. However, sticking to the majors usually is more hassle-free.