Peter Driscal's parents are very protective of their son and obviously want to protect him from Cole—a.k.a. the monster who hurt him. That said, even though they want to keep Peter as far away from Cole as possible, they are also realistic about Peter's issues and the fact that they need to do something drastic in order to keep him from committing suicide.
In the end, Mr. and Mrs. Driscal bring Peter to the island to join Cole, though not before warning Cole that they'll destroy him if he so much as lays a finger on their son again. It's clear that they're good parents who care deeply for their son and will do anything to take away his pain, even something as unorthodox as leaving him alone on an island with the guy who messed him up so badly in the first place.