When Aguirre observed the boys with his binocuars (at least once), he had done that for 10 minutes and waited until they got dressed before he went to tell Jack about his Uncle Harold having pneumonia.
In the movie, when Aguirre was watching them, the guys were very happy just being themselves as immature teenagers. In the movie, Aguirre certainly waited quite a while before he went to deliver the message from Jack's mother about his uncle. I think that, in a way, Aguirre had some respect for the guys private time together, although it was literally in the open.
From the way that the story goes, Forest Rangers up in the lookout towers belonging to the US Forest Service could have observed what was going on, too. The book does not mention that fact though. But, even when the guys were up on the mountain, at the campsite and with the sheep, they could see the lights of vehicles on the highway down below.