SPOILER***
To me it looked like Harry was pretty much dead after Venom threw him off the building, he was stabbed pretty bad so I didn't think there would be that moment afterwards where he was lying there still alive with Mary Jane there, so perhaps Peter thought he was already dead. He didn't have a chance anyways, and it's not like Peter could take a moment away from Venom to go talk to his dying friend.
To me it looked like Harry was pretty much dead after Venom threw him off the building, he was stabbed pretty bad so I didn't think there would be that moment afterwards where he was lying there still alive with Mary Jane there, so perhaps Peter thought he was already dead. He didn't have a chance anyways, and it's not like Peter could take a moment away from Venom to go talk to his dying friend.