Welcome back to the Wefugees Community and thank you for reaching out to us.
Since for the naturalisation an applicant has to prove that he/she is ensuring the means of subsistence, the authorities probably have doubts that your friend can secure his livelihood in the future while having to pay all his debts. However, please consult the lawyer to know for sure about the reason for rejection (at the end he/she gets paid for taking care of this) - every case needs to be assessed individually.
I hope this helps and don't hesitate to get back to us with any further questions.
All the best,