Influence is a sketchy thing to define. What is influence? Is it the measure of impact a person has on others? Or is it just how willing the person in question is to follow the influencer? In the end is doesn't matter, only that it is the people who you know the best who have the greatest influence upon you. A parent will influence how a child grows up and treats others, but a friend would also influence how the person in question interacts with other people. Obviously I have no choice but to disagree with the point of view of this passage.
Though a parent might not have the greatest influence upon a person, a parent has great influence upon their offspring's behaviors to their own children when they have a family. If a child is left alone for a majority of the time, he/she grows up thinking that since they turned out alright, their own children will be able to cope. Other children however do the opposite to what they experience when they were young!
Social behavior is influenced by a person's friends. Who he/she interacts with, gangsters or friendly people, defines how they interact with others. Parents often don't influence a child greatly in this area. Sure, maybe your parent speaks politely to everyone they meet, but you talk like you words came out from the trash! Obviously the way you interact with people heavily depends upon personal peference and possibly, future occupation.
A person is influenced by whom they want to be influenced. It doesn't matter how long you spend with a person, or how much you know a person. In the end, it is how open a person is to others that determines how they are influenced.