Debt Collectors Stalking, Publicly Shaming People Through Facebook
wtsp.com
wtsp.com
That they used Facebook is irrelevant -- calling the sister, emailing the sister, or sending the sister messages via carrier pigeons are all equally illegal. And if they were stupid enough to post a message on her wall about it, which I doubt, statutory damages would hit six figures very, very quickly. ($1,000 for every Facebook friend who is not your spouse.)
They do that to try to get in direct contact by tricking the family and friends into giving better contact info.
Yes, debt collectors are rude and often unethical... But I have little pity for people who try to avoid them. The problem is not the debt, but the avoidance of it.