You are completely correct!
In many ways, an avulsion fracture is like a stress fracture, in that it's not a "real" break and treatment requires rest, icing, and range of motion exercises. As a runner and running coach, I'm very familiar with stress fractures, and while they keep you from training for a set period of time, they all heal on their own, and the affected hairline area is stronger than it was prior to the injury. With an avulsion fracture, it too will heal on it's own, unless the bone fragment has moved too far off of the source bone and then surgery is required - this is easy enough to see with medical imaging - therefore, if he hasn't gone under the knife, then it wasn't spaced far enough away to require it. This is also the reason the initial diagnosis would have been incorrect - the bone fragment had not pulled far enough away to be discernible on the medical imaging, thus they assumed it was a groin strain (I believe that was the original diagnosis).