Just bought this guitar. The previous owner said, he polished the frets with steel wool when he was younger and managed to remove some of the finish off the fretboard. I guess he didn't tape off the fretboard.

So now I have this guitar and wanted to know if there was a quick fix for scratched maple fretboards.

In a direct answer to your question, no. Consider too, that grime has permeated into the pores of the wood where it goes below what you see on the surface. Consequently, the only way to get rid of it is to remove and replace the 'infested' wood which obviously, is not a practical option. And no, bleach, etc. may lighten it but it will only come right back in the future.

Yes, that can be fixed. You have probably 2 choices: 1) depending how deep the scratch is and how much it bothers you, you can have a luthier remove the fret and sand the scratch, make a spot finish and buff the neck. OR 2) remove all the frets, sand and refinish the entire neck, and put new frets on. It all depends how much you like the guitar and how much you want to invest in repairs vs. trading it in and buying a new one. If it was me, and I liked the guitar, I would have all the frets removed, neck resurfaced and re-finished and place stainless steel jumbo fret wire on it. If you do this, your next fret dressing would probably be in 20 years, and the stainless steel frets will out live you.