I don't have a cite for you, but my understanding has been---so long as you can document that you sent out the notices, advising the customer of the RMD, and you have properly informed the IRS that the account is subject to RMD, you have fulfilled your legal obligation with respect to the RMD.
You can not force the client to take it. If they don't it's their penalty and their problem.
That said, from a customer service standpoint, if you have one client or a small number of clients in this type of situation you may wish to make personal contact with them and discuss the potential issues involved (of course, being catious not to dispense tax or legal advice).
If you have a large number of client in this situation, you might wish to take a look at your communication pieces and the letters that are being sent out. Are they clear, understandable, and written in "plain english" (as opposed to legalize).
You might also wish to consult with your bank counsel on the matter and see what suggestions s/he may have.