That is right because those people know how they can abuse the rules from many ways although casino can find out who are their members that abuse the system on the casino. If my friend borrow my personal document and only for verifying his account so he can get the bonuses, I will not give my document even if he say that he will give the bonus with some money as a gift. I will not tempting with that because I risk myself and him from doing that. I will suggest him to search from the other friends who wants to do that. I don't know if that will be safe for me or not but my personal document can be on the wrong hands.
Good advice by suggesting him to find someone else to borrow his documents, I myself certainly do not lend documents to just anyone, only to people I trust, as I have said it is indeed quite risky but I do not forget to always consider it first because I myself do not want to experience problems with my personal data.
With what you do when a friend wants to borrow a document to verify and you do not lend it, that is the right action, because that way your documents will be safe, keep it. Maybe I myself will try to refuse my friend who borrows my documents later.