What should I do concerning a non-Muslim person who owns a keychain engraved with the word of majesty (Allah)?

There is no harm in a non-believer touching a keychain on which the majestic name of Allah is written, because the Prophet, may Allah’s prayers and peace be upon him, sent a letter to Heraclius containing the Basmala and a verse from the Quran. It is permissible for a non-believer and one who is in a state of ritual impurity to touch one or a few verses along with something other than the Quran. This is especially true for someone who glorifies Allah the Almighty.

As for hanging something that contains the remembrance of Allah and using it as a keychain, it is not appropriate, as it may expose it to disrespect, falling, or entering the restroom. However, it is not obligatory to change it. One should focus on inviting the non-believer to Islam, for this is what will benefit and save him.