The situation will vary, depending on whether there's equity in the property...whether the person is selling for more than is owed or less.
Let's first consider the situation where there is equity. Someone owes $200,000, but the house is worth $500,000. In addition to the $200,000 mortgage, they now owe $10,000 in back payments, interest, and lawyer's fees. And let's say other transaction costs (including real estate agent's commission, if there is one, payable by the seller of course, would be $15,000). Whether it's listed with a Realtor or not, you can make any offer that'll cover the outstanding mortgage, delinquent payments, interest, fees, and any other required transaction costs. So you might offer the seller $250,000. Or $300,000. Or $350,000. It's your call. The seller would receive that, and it'd be enough to pay off the mortgage, eliminate the delinquencies, and (if listed) pay the Realtor's commission.
It gets more complicated if all those mortgages, fees, and expenses add up to more than the offer made on the house. Take that same $500,000 house. But let's say the owner bought it last year for $600,000 using 100% financing. The house's value has since declined to $500,000. Plus, the owner's delinquencies, penalties, etc., are $50,000. So he owes $650,000 on a house worth $500,000. In this case, the seller has to consider a "short sale"--selling for less than is owed. Because the lender is "coming up short" on the deal--losing money--the lender has to approve the transaction. So you might decide to offer $450,000 or $425,000, or something else. You certainly wouldn't offer over $500,000...that'd be more than the house is worth.
Short sales are complicated and time-consuming. And they're uncertain; you don't know whether the lender will accept your offer.
That's it in a nutshell. Good luck.
If the house is going into foreclosure and is not listed for sale, contact the owners and make them an offer. If I were going into forelcosure, selling would be a great way out of a bad situation.