How can one undo the magic spell cast on a girl through a ring while she is now restrained? And does the "warning" apply to the one who performed the act and his mother, knowing that his mother met her and nothing happened, and he personally did not meet her?

You have committed a great error by practicing sorcery and seeking to perform it. This is among the destructive sins and necessitates disbelief.

You must repent sincerely and seek forgiveness from the girl you wronged. If it is not possible to disclose the truth to her, to avoid a greater evil, then praying for her and seeking forgiveness for her will suffice.

If it is possible to find the sorcery, destroy it while fortifying yourself (with protective prayers), and try to perform ruqyah (Islamic exorcism/healing) on her with legal ruqyah to remove its effects.

The verse mentioned has no relation to the matter; it urges disbelievers to learn a lesson from the state of the people of Thamud. The Prophet (peace and blessings be upon him) warned against entering the dwellings of those who wronged themselves.