In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear questioner! Thank you for this question, which reflects a true desire to increase your knowledge of Islam.
In fact, Islam establishes zakah to act as a sign of social solidarity between the members of the Ummah, a token of gratitude to Allah, and a means of purifying the self from its greedy desire for money and wealth. Muslims can also give sadaqah (voluntary charity) to gain even more merits and reward. By doing so, they show their love for their fellow Muslims, care for their kith and kin, and have a sense of belonging to the tapestry of the Muslim society.
Responding to the question, Dr. Rif`at Fawzi, former professor of Shari`ah at Cairo University, states:
If the money given to that person [by the prison authorities] does not suffice him, then you are permitted to give him some money from the zakah money. You can give him up to suffice his needs. This is the maximum that you give him what suffices his needs. However, you cannot give him money to be used in things that are unlawful. So, for example, you cannot buy him cigarettes and stuff like that. You can give him money only that suffices his needs and makes his life easier.
Moreover, Sheikh Ahmad Kutty, a senior lecturer and Islamic Scholar at the Islamic Institute of Toronto, Ontario, Canada, adds:
If this person does not have the sufficient means to support himself or his family (while he is in jail), you are certainly allowed to give him from the zakah. While giving him, you should not give more than the amount necessary for maintaining a satisfactory standard of living as determined by the standards of the society you are living in.
Almighty Allah knows best.