This article explains how to set a ringtone for someone on an iPhone running iOS 12 or iOS 11.

Assigning different ringtones to specific contacts in your address book is a fun way to personalize your iPhone, and it helps you know who's calling without looking at the screen. To personalize the ringtones for your callers:

Tap the Phone app to launch it.

In your contacts list, find the person whose ringtone you want to change from the default. You can do this by searching for their name in the search field at the top or by scrolling through the list.