I can't see that it's a huge boundary violation. That entirely depends on what you post. I am FB friends with a lot of my veterinary clients. All I post on my FB page is feel good pics of my dogs/horses, interesting veterinary articles, etc. Several of my friends who are doctors do the same. FB is a fairly superficial medium. If all he has on his FB is pictures of his favorite hobby or sports teams and such it's not a big deal