My husband sold his 40% share in a restaurant early 2015.

My husband sold his 40% share in a restaurant early 2015. The following year (last financial year) we got a tax bill of around $40,000 for capital gains tax.
The restaurant then had several different owners sell their % of shares in the business. The most recent purchaser has come back to us claiming that the business was over drawn (I assume whilst we were a partner) by $40,000 and wanted an invoice for this amount. The partner we had at the time assured us it was nothing and he'd take care of it, as he'd always been the one doing wages and books etc.
My husband has barely worked the last financial year as he is waiting for a kidney transplant. He earned $10,000 overseas before he came back to AU sick in November last year. Due to this "invoice" that had to be raised, it has now been said that he earned $45,000 in restaurant consultation, and of course no tax has been paid in this amount so we've been given a $9,500 tax bill which we will struggle to pay on top of the $40,000 we are already paying off in instalments!
My question is, how can this come back to us over 18months after the sale, which supposedly covered all outstanding bills etc. an if so, how are we liable? Did the partner doing all the bills and books have a duty of care to alert is to this? To cut back on distribution to avoid this much overdrawn? If we were only 40% share, surely the business wouldn't have been $100,000 overdrawn when we sold without us knowing?
I'm sure this requires more information and investigation than I've supplied you with but is it worth my trouble investigating or is it likely to be bad luck, you're liable regardless?
Appreciate some advice,
Leigh***** *****