When Ed called to ask Ron to come back, he didn't tell Ron where to go
On the phone, Ed didn't tell Ron to go to the zoo. He just asked him to come back. I know Ron's pals ended up taking him anyway, but there was no way for Ed to know the guys were there. Ed simply didn't tell Ron. Writers messed that up. No way was it intentional. When you call someone up from a random location and ask them to meet you, obviously you tell them where that location is.