Disclaimer: I have no experience on using the Deck of Many Things, either as a DM or player.
The description of the Donjon card from the Deck of Many Things says:
You disappear and become entombed in a state of suspended animation in an extradimensional sphere. Everything you were wearing and carrying stays behind in the space you occupied when you disappeared. You remain imprisoned until you are found and removed from the sphere. You can't be located by any Divination magic, but a wish spell can reveal the location of your prison. You draw no more cards.
The description is vague and does not exactly tells what to do to rescue the victim, except "you must be found and removed from the sphere". It does not say how to find the victim, how to remove them from the sphere, and other limitations.
For example, does the prison lie on a specific plane, which can be accessed by the plane shift spell? How can I break/remove someone from the sphere? Is it simply by going there and grabbing them outside their original position?
How does freeing a victim from Donjon work?