I would disagree with scoring it as a FC.
If you were equating this scenario to "defensive indifference" on an attempted steal (in which FC IS indeed the correct scoring notation), it's not the same. In a defensive indifference situation, the defensive team KNOWS that the runner is attempting to steal and simpy LETS him do it. That's not what happened here.
That leaves two possible choices:
1. If in the scorekeeper's judgment the runner would have made it safely to 3rd base even if there HAD been a play on him, then it's a simply a successful tag-up situation.
2. If he feels that the mental mistake made by F8 allowed him to advance, then rule 10.12(a)(1) gives him the right to score it as an error. Typically, mental mistakes are not subject to being scored as errors, but there are some exceptions to that -- and in my view, this is one of those situations.