If the deal makes sense and meets your goals (CoC, cash flow, etc...) you should make the purchase. You should ask for the leases and read them to see what exactly you are inheriting to better understand what you are on the hook for.
As far as the past due rent, not sure why the owners think that you should be on the hook for them not enforcing a lease agreement properly, but maybe you can make a deal with them that passes on the $200 per month as long as he pays it.
If you do re-sign this tenant with the $200 as part of the terms of the lease - this will be reflected as a loss of $200 per income each month when fully repaid on your books. Anyone looking at your books might want to know why you suddenly decreased that units rent by $200 per month.
Either way -- I suggest you read the individual leases (not just one as they may be substantially different) to see how you can be in a position to get rid of the chronically past due if needed as well as frame for a future need of getting rid of the gentleman who is "getting back on his feet" in the event that he stumbles again.
Good luck!