All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
There are certain conditions for the validity of the lease ending in ownership agreement, and we have previously underlined them in fatwa 321706.
In any case, it is impermissible to sell the property which is leased through a lease-to-own contract because the lessee (your friend in this case) does not own the property by merely conducting the lease contract. Al-Ma‘aayeer Ash-Shar‘iyyah (Sharia standards) reads, "In all cases of ownership based on a promise of gift-giving or a promise to sell, the ownership contract must be concluded upon fulfilling the promise. The ownership of the property is not automatically transferred (to the beneficiary) merely based on the initial promise document."
Although it is invalid for the lessee to sell the leased property because he does not own it at that point, it is permissible for him to lease it out to another because a lease, in essence, is selling the benefit of the property, and the lessee owns that benefit. Al-Ma‘aayeer Ash-Shar‘iyyah by Accounting and Auditing Organization for Islamic Financial Institutions (AAOIFI), regarding the lease ending in ownership contract, reads, "It is permissible for the person who leased a property to lease it to other than the landlord in return for a payment equal to, greater than, or less than the specified rent, to be paid in advance or as a delayed payment. This is known as sublease; it is allowed unless the landlord has stipulated that the lessee must not lease out the property to another or should obtain his permission first."
Allah knows best.