I can see three options here.

1. Wait and hope @DestructibleLemon fixes their answer. They still have 5 days to do so.

2. Award the bounty to another answer that fulfills the criteria. @Riley's seems to be the fastest valid answer, and that an answer has to be valid to be posted on PPCG, bounty or not, is always implied.

3. Ask a moderator to refund the bounty, either publicly or via a flag. We would do so for a bounty on a question that should be closed; this situation isn't that different.

What option you choose is up to you.

That said, I'd like to add that FGITW bounties, at least ones without any quality thresholds, seem to be a bad idea. They encourage subpar answer for the sake of collecting the bounty, which isn't doing anybody any favors and leads to problems like the one at hand.

I can see three options here.

1. Wait and hope @DestructibleLemon fixes their answer. They still have 5 days to do so.

2. Award the bounty to another answer that fulfills the criteria. @Riley's seems to be the fastest valid answer, and that an answer has to be valid to be posted on PPCG, bounty or not, is always implied.

3. Ask a moderator to refund the bounty, either publicly or via a flag. We would do so for a bounty on a question that should be closed; this situation isn't that different.

What option you choose is up to you.

That said, I'd like to add that "Fastest Gun In The West" bounties, at least ones without any quality thresholds, seem to be a bad idea. They encourage subpar answer for the sake of collecting the bounty, which isn't doing anybody any favors and leads to problems like the one at hand.