It's so not true! I know people who transfered to SI during soph or junior years, and they've made friends pretty quickly. Thing is, there is quite a bit of drama, and people tend to switch cliques every once in a while. When transfers first come to the school, they get assigned a host who's in a few of their classes. This way, the transfers can better know the school and activities. Personally, I've met a lot of people through activities, and it's not hard to make friends. The more involved you are, the better it is to make friends. There's no real difference. I didn't hang out with my current friends until the last month of junior year. We're a very friendly community (most of the time).
Good luck to him. I know quite a few freshmen since my brother and his friends are in that class.