Without being able to see the head and limbs, I would go with a western pond turtle too, however, old male RES that are starting to go melanistic look pretty much like the turtle in these pictures. Ask your friend to put it into a tub of water - a light colored tub so we can see through the water - then take more pictures when he brings his head out.
Western pond turtles don't have the serrated edge on their shell like this one, and they also have very long tails. It looks like this one has a big fat male tail, but not too long.
Agree with melanistic red ear now that I'm on my computer instead of iPhone and can see it better. Don't see how those rear marginals could be Actinemys. Plus looks like the front claws of a male red-ear.