The IRS is going to go by what they think they sent, regardless of what your client may or may not have received.  If that's different than what was claimed on the return they will "correct the math error" so you likely do not need to amend.

This whole thing is another mess and is eerily indicative of today's political climate when both sides can't even agree on what the facts are!  Maybe the IRS sent it out, maybe it was lost/stolen, maybe the client got it, maybe an ex-spouse got it, who knows!

For anything already filed, I would wait for a letter from the IRS (or for the clients to get their refund if they are expecting one--they probably are, who files early in the season if they owe money, right!?!)  Kick this can down to the third week of April (or take some time off and start reviewing these things in May!)

Rick