Is it permissible to help a friend who has accumulated debts due to sins he has repented from, but who does not exert enough effort to repay them, and requests monthly financial assistance from the helper?

Your eagerness to help your friend repay his debt is commendable in Islamic law, for the Messenger of Allah, peace and blessings be upon him, said: "Whoever relieves a Muslim of a distress from the distresses of this world, Allah will relieve him of a distress from the distresses of the Day of Resurrection. And whoever eases [the burden] for one in difficulty, Allah will ease [his burden] for him in this world and the Hereafter." If your friend will borrow an amount from you and repay it in installments without any increase, this is permissible. You should document the debt in writing and have witnesses. Lending him is not obligatory upon you, but it is advisable to offer him advice and guide him to avenues of lawful earning.