decision on her own. It sucks for the daughter and will probably ruin their relationship if they have one and the mother will probably get sued if she sells it out from under her daughter, but oh well. I would never buy someone else's known liabilities especially if I knew the business was in decline. You are asking for trouble. They either need to clear up the liabilities before the sale (with proof of such) or sell you the assets only and GTFO. I'm sure your lawyer and accountant would agree.
....i will add that having failed and successful businesses in the past - usually when a business is at this point, the "mom and daughter" certainly don't have income or assets to be able to pay off debt they have racked up. The chance of them being able to clean up liabilities is probably less than 10%.
