Our answer follows. (Thanks, Ron!)
NJMC Naturalist and all-around turtle guy Brett Bragin says:
"It's kind of hard to tell from that photo. At first glance it looks like an Eastern Painted Turtle, but the head and neck is quite dark and seems to be lacking the yellow or red blotches that they "normally" have. I also do not see any color on the marginal scutes that the painted turtles usually have."
When informed the photo was taken in Kearny Marsh, Brett added: "I would go with painted turtle (although it could be a slider, but again the photo of this beast lacks any of the distinguishing colors that allow you to be certain. It is also hard to tell the size of the animal. Next time, tell them to sneak up on it and put a ruler behind it before taking the photo).