When the sheriff set him free...
Why didn't Emily Blunt and the Indian ride off together?
The sheriff said something about the guy having to disappear, but why couldn't the couple have disappeared together?
Not only that - but they could have just stayed there and been together. No reason for the Sheriff to split them up and force them to go separate ways. The Sheriff made-up a believable story and didn't need to convince anyone else of it.
The only other witnesses were the mother/boy and I don't think they would be at risk of saying anything.