It sounds like they are wanting the home owners to create a "Cash Note". This is fairly common - especially now. I would only suggest that they are working with a reputable company.
If it keeps foreclosure from their credit - it might be a pretty sweet deal. Especially interest free?! That is great!
Just advise them to make sure all of their i's are dotted, and t's are crossed.
Edit: Well, I read the other responses and it seems pretty clear that they should call their mortgage company. But, chances are, if they are in this situation - they already have called them and the mortgage company has let them down.
What these "we buy houses" people do is buy a house for very cheap - so they can still make a profit on them. That is just good business sense.
What is sounds like from your friends case, is that they owe more on their mortgage than what the "we buy houses" people can afford to pay them... so... they (the we buy houses people) are going to pay off their mortgage, and in turn, your friends are going to sign that promissory, or cash note to pay those we buy houses people for their loss.
This is a very creative way to get out of a sticky situation. It is just important that they are dealing with an honest company - and yes, those honest companies do exist.
Of course, if their mortgage company will help them out - that is even better. But, I am willing to assume that they have tried that already to no avail.