welcome tinkerwell and congrats to your friend!
I can't explain it because I've never seen anything like it. I've poured over the photos and don't see anything that doesn't look like native rock. Perhaps at one time it showed some tool marks but was in the river for a very long time. I knew an old man, growing up, that made fake artifacts. He would put them in the springhouse tank for a year or more and they'd get really smooth and aged looking. Tell us more about what the archie said.
I say "BANNER"