he is absolutely entitled to legal advice, but I can't think what they would tell him other than what I have said. at the IUC he will be advised that he can get legal advice and if he doesn't like the way the interview is going he can stop the interview at any time. the interview will be taped and he will be cautioned, but I can't see how he can be in trouble if he has some evidence.
tell him to try to make a list of details of all the cash payments he can recall. they have the authority to check the other couple's accounts, but I guess they won't have paid the cash in anywhere.