Guy speak is not that complicated, cool means that he thinks the girl is likable and fun. As far as non-sensical, if he uses "cool" as some sort of verbal twitch where everything is cool, then perhaps it meant about as much as clearing a throat, other wise its a compliment. He could see her as a friend or could be interested in more, this comment says little either way, the person to ask would be him.
well it depends, so far it sounds like he's talking in a friend-like way, but he can possibly mean that he is into her, its very complicated and I would need more deeper information to give a better answer such as who he told that the girl is "cool" and whatelse was he talking about. If he told a friend that the girl is "cool" it is more likely meaning he is interested...hope I helped