My believe in AP parenting too, like your friend, and my infant is very confident now. He used to be very clingy (although I'm not so keen on that word), but I found that was more frustration and the pain of reflux than anything else. Now he's been crawling for a month and and is getting faster and more experienced, can pull himself up on things to reach what he wants and is on new reflux treatment it has stopped. He still comes over to me and asks me for cuddles, by pulling on my leg. But I think that is lovely....
He understands phrases like 'No' and 'Let Go'. Which is responds to positively most of the time.
I personally think AP tends to bring out the opposite, but all babies are different at the end of the day, and have differing personalities.
They say between 12-18 months, a baby can recall a mental image of the most familiar caregivers. This image helps to provide a secure base so the infant can begin to move more easily from the familiar to the unfamiliar. The mental presence of the mother allows the infant to, in effect, take mother with her as she moves further away from the mother to explore and learn about her environment. AP children show less anxiety when moving away from their mothers to explore toys. These babies mentally and physically check in with mother for reassurance and a familiar ";t's okay' to explore. The mother seems to add energy to the infant's explorations, since the infant does not need to waste energy worry whether she is there. During an unfamiliar play situation, the mother gives a sort of "go ahead" message, providing the toddler with confidence to explore and handle the strange situation. The next time the toddler encounters a similar situation, he has confidence to handle it by himself without enlisting his mother. The consistent emotional availability of the mother provides trust, culminating in the child's developing a very important quality of independence: the capacity to be alone.
One without this can lack confidence that his attachment figures will be accessible to him when he needs them. He may adopt a clinging strategy to ensure that they will be available. Because he is always preoccupied with it or else spends tremendous energy "managing" without it. This preoccupation hinders individuation, exploration, and possibly learning. In essence, the attachment-parented baby learns to trust and develop a sense of self. These qualities foster appropriate independence. Studies have shown that infants who develop a secure attachment to their mothers are better able to tolerate separation from them when they are older. As one mother of an AP child said: "He's not spoilt; he's perfectly fresh!"
But this is my argument to what you said. I know some would argue that because a child finds it quite normal to cope without his Mother being there, he finds it easier to explore ad be on his own.
My experience will be my instinct my experience will be learning whats right for my baby ... my experience will be looking at how other Mums do things taking that in and keeping in mind.
, there is a bit of diffenace in letting a baby cry itself to sleep in upto 5 minutes than leaving it all day, so it learns that no one comes!!! 
