Is it permissible to learn how to solve or undo magic spell from someone bewitched?

It is not permissible to learn magic to undo it or for any other purpose. Rather, it is one of the nullifiers of Islam, because it can only be learned by falling into polytheism (shirk) and worshipping devils through slaughtering and vowing to them. This is the "mutual benefit" mentioned in His exalted saying: "And the Day He will gather them all together, [He will say], 'O company of jinn, you have greatly exploited mankind.' And their awliya [friends/supporters] among mankind will say, 'Our Lord, we benefited one from another, and we have reached our term, which You appointed for us.' He will say, 'The Fire is your residence, wherein you will abide eternally, except for what Allah wills. Indeed, your Lord is Wise and Knowing.'"