The individual is going to have to take legal action against the person who submitted the report, if and only if the claims made are untrue. RipOffReport is notorious for refusing to take down submissions made by people and--even worse--they do not allow people to edit or remove submissions they made. So if someone posts something in anger, it is up and there is nothing they can do about it if they have second thoughts. RipOffReport has a high pagerank and ranks high in Google. There really isn't a whole lot you can do.
Maybe your client should have some contractual language requiring people to deal with him/her or seek arbitration in cases of disputes instead of making posts on sites like RipOffReport.