Hello desertrose and welcome to the Dubai forums
I can only suggest possible sources which might have information, I don't know how easy it is to be 100% sure unless you have a direct contact with someone in one of these places.
Emcredit - I don't know much about them but I think they monitor credit profiles in the UAE
The bank in the UAE where your friend had debts
Dubai Police (or Abu Dhabi, Sharjah, etc).Dubai immigration
- I expect they will only have information if there is an entry ban, or if the police have asked to be informed when someone goes through immigration.
Of course, a problem with contacting any of the above is the accuracy of the information you receive. It will be difficult to know if an answer given is 100% correct. A good warning sign is often when someone says "No problem, no problem". In my experience that usually means there's a problem but they don't want to say what it is.
My understanding of debts in the UAE is that having a debt in itself does not result in detention, it's the presentation of a cheque that subsequently bounces that results in a police case and possible jail time (and/or being held up at immigration). Which is why banks take security cheques off their customers when loans and credit cards are provided.
One more thing about blacklisting, it seems to be a generic term used to describe different situations. A UAE entry ban blacklist
usually only results after a conviction, a bounced cheque doesn't result in an entry ban (which makes sense - the bank would like the debtor to come back and pay it off and/or go to jail).
It might be worth trying to contact the bank where the debts were to see if a repayment schedule could be arranged. It would be in their interest not to pursue the matter with the police in that case, after all if someone is in jail, it's more difficult to earn money to repay their debts. During the course of that conversation you might be able to find out what actions the bank has taken so far.
Good luck and let us know how you get on