I'm too embarrassed to show my team leaders code to other developers..

unreadable, no tests, parameters hardcoded..

But managers want to "help him grow"..
When do you decide you can't fix a lemon?

Probably at acknowledgment of said lemon being a lemon.

“When life hands you lemons, just say fuck the lemons and bail“

You help someone grow in a junior role, not while they're leading a team.

I'd probably retort with "ok, when can I have my promotion to help me grow, even if I'm unqualified?!"

I’d say look at this as an opportunity. I was once asked to do something similar - I would stay late and help the person with their assignments. Designing solutions, refactoring, debugging, etc. Poor guy just didn’t have a head for writing software. Well, he was very appreciative, and it turned out he was related to some very high ranking folks working at the company (I didn’t know that at the time). Worked out well for me. In any case, it can be beneficial to help someone out even if you think it’s not worth it right now - it will show your superiors the type of person you are.

@fiftyhz I think this is a great point.
Some people don’t have the mid set for programming but who knows, this guy might turn out to be a shit hot people or project manager and then entrust the technical leadership to you.
I’ve been in a scenario where I had a great non-tech manager and we had built up trust where that manager would back the teams decisions to the hilt. Best team I’ve ever been in.

Also you don’t know what the future holds, a few years down the line he might move on to somewhere else and then offer you a great role there. Or maybe not.