I believe the reference is to the 5th Circuit case, the decision in which should come any day now. However, because the unconstitutionality argument is based on the repeal of the individual mandate tax as part of TCJA, and the repeal was not effective until 1/1/2019, the complete unconstitutionality of the ACA (if that's what the 5th Circuit holds, and there are a lot of other ways it could come out) will not reach back earlier than that. In any event, a decision by the 5th Circuit that the ACA is now wholly unconstitutional would be appealed to the Supremes and a 5th Circuit judgment of unconstitutionality would be stayed during the appeal.
I should add that the IRS isn't any better at getting blood from a turnip than anyone else. If all else fails for your client, jireh87, and you get the assessment, you can always try to negotiate over ability to pay. If an ALE neither provided coverage nor reported, then you face both the ESRP and 6721 and 6722 penalties. You might get the penalties mitigated. I suspect the only way to reduce the ESRP is based on ability to pay.