My advice short: Fire Fred. Long: Depends on your process. Generally you should have refactoring tasks. So a dev detected bad architecture or some issue in the ticket. Then do the stuff and make a refactoring task. Or change the ticket. This way Fred would have enough of work that he loves. Sometimes he should do also stuff which he hates - its a job.

I said fire because it reminds me of a coworker who was very lazy. He complained about code (yes it was messy) but he was extremly slow in delivering. Simple tasks took 5x the time they should take and the refactoring wasn't that good. I mean 2x the time might be ok if you improve stuff. But no way a 5x delay. Especially in a large project, if there are some really messy things, you need to talk with other devs to decide on architecture and so on. It's no way a one man show.