The book states that children who were securely attached as infants are more likely to have close friends as teenagers. It is true that secure attachment correlates with close friend relationships later on, but other factors may play a part in that result. It may also depend on if the infant was allowed to be with and play with other infants and their opportunitites to be with peers their age as they grow. More exposure to their peers as they grow can positively influence their friendships as teenagers.
I agree with this because it definitely shows how attachment does have an influence, but there are other aspects to the attachment and environment, as in if they are exposed to a lot of other children, etc., that greatly affect a child's well being in the future.