Stripped/bent threads on outdoor chair leg..suggestions?

If you can borrow a tap, that would be the proper tool to repair damaged threads on the leg of the chair. If you can't borrow one you can make a homemade tap. Get a bolt that matches the threads on the foot. Use a cutoff wheel on your dremel to to make a cut on the bolt that runs across the threads (lengthwise on the bolt). Use that like a tap to cleanup the threads.

This relief you cut in the bolt creates a space for the deformed thread material in the chair to get out of the way. Since you are just cleaning up an existing thread (rather than creating a new one) there shouldn't be that much material which is why this works. Just go slow make a little progress, back it out to clean out the metal bits, and go again. Since the foot was screwed most of the way out it is probably just the first few threads that are a problem. Don't cross thread it or you will make it worse.

A tap would be best, but if you have the same size stainless or a hard Grade 6 or so bolt, you could try threading it in gradually with a couple of drops of oil.

Tap for larger bolt. Then use larger bolt

use something smaller diameter to try and straighten the threaded "nut" inside the chair leg, like an awl.

Otherwise i'd drill out the chair leg, or shorten the threads on the foot screw and JB Weld it in place.