Here's a book recommendation for your cousin. It goes into detail about the whole process in which male intimate abusers select, target, groom, and eventually abuse. The frog in the pot of hot water is a perfect analogy but there are early warning signs. They probe boundaries from the start and figure out where your vulnerabilities are. Smart successful women don't see it coming. They are used to working hard for what they have and they are easily convinced that investing more effort into a relationship will yield results because that works well in other areas of their lives. The book is called "How He Gets Into Her HeadHow He Gets Into Her Head"
He's not a skilled abuser yet. He's young. He'll refine his technique over time as he continues forward in intimate relationships. This book goes into great detail about the whole process.