You could prepare a paper 1040X  and provide it to the client to mail in, along with a letter including instructions on how to file.

Were it I, in the letter I would make it clear, that should they decide not to file the amended return to correct the error on their return, they would be choosing to terminate the relationship with me, and that I would not be able to provide tax services to them in the future, if they were unwilling to take all the necessary steps to make sure that their returns were filed correctly.

Tough situation, best wishes on a good result.