What are the options for a broken business contract?

My sister sold her company two years ago, with the payments being monthly percentages of the new owners earnings. After four years, the remainder of the debt was to be paid off in full. Now, halfway through, the new owner has asked her if she would accept a lump sum payment for about 1/3 of what he owes her. He is claiming health problems and that he is going to need to downsize or resell the company. He is making it seem that if she doesn't accept this offer, than she won't get any money at all. They had a contract, but since they had been friends at the time it was never looked over by a lawyer. It was mainly forms that were downloaded off the internet. The company was made into an LLC, so does that mean that his personal ets cannot be taken? Obviously she is considering taking him to court, but doesn't know if it will be worth it if she has to spend lawyer fees and then won't get her money. Any advice???