All this shows is that at 16, 17, 18, whatever, you're too young to disconnect emotional attachment and physical intimacy, especially when you're still at a stage where you're discovering intimacy. It's already hard enough when you're an adult.
Your friend clearly likes the guy but was convinced, like a fool, to try a casual relationship. She probably likes the guy quite a bit and having slept with him is not going to help things.
Even when you're an adult, it requires a certain amount of emotional detachment to just have sex as sex. Why are a lot of pornstars, strippers and prostitutes women who were abused as children? It's because emotions and sex are related, even when you're trying to separate the two. A lot of girls who sleep around have unhappy family situations, lacked affection.
Some people try to avoid mixing emotions and physical intimacy, because of emotional issues. So in a way, emotions never disappear. We just sometimes attempt to ignore them or suppress them, as best we can.
When you're still a teenager, being naked with someone, being intimate, kissing... is bound to lead to feelings developing.