10.21—Knowledge of client's omission.
A practitioner who, having been retained by a
client with respect to a matter administered by
the Internal Revenue Service, knows that the
client has not complied with the revenue laws of
the United States or has made an error in or
omission from any return, document, affidavit, or
other paper which the client submitted or executed
under the revenue laws of the United States, must
advise the client promptly of the fact of such
noncompliance, error, or omission. The
practitioner must advise the client of the
consequences as provided under the Code and regulations of such noncompliance,
error, or omission.