You said inherited...If that is correct, the stock basis would be as of the Date of Death of the person who's estate distributed it to your Client. This may or may not be correct as the date acquired.

If their Investment advisor won't help, I have had luck working with someone local that I have a relationship (Investing with) to get the information.

BTW if their advisor won't help, I normally recommend that they promote him to their ex investment advisor.