is it a problem to buy an iPhone in a different country?
There are two issues:

First, while Apple's warranty is international, Apple sometimes limits overseas service to the country where the device was purchased, or requires users to return the item to the US for service. This stackexchange answer has more information.

Second, she must be careful to purchase a model which will cover the phone bands she expects to use. iPhones sold in different countries sometimes have different coverage, and often multiple models (band coverage-wise, not "7" or "8" or "X" - wise) are offered. This Apple webpage, for example, lists the models that will provide LTE coverage as well as the bands covered. (I know it's bad form to refer to an external page, but the info is far too large to copy-and-paste into this answer.)