It would certainly be illicit for him to be consecrating a host though would it not?
Maybe. Maybe not.
Remember, the word “illicit” means something contrary to the law. Something illegal.
The laws of the Church only bind those who are members of the Church (unless by their very nature they apply to non-members).
We’re discussing a former Catholic person here. Once a person formally leaves the Church, he is no longer bound by the Church’s laws. We’re discussing a hypothetical situation here—and the issue of “how” he left the Church has not been defined (as a hypothetical, that can be whatever the author makes it).
I know there’s a popular phrase “valid but illicit” that often gets used when talking about sacraments celebrated outside the Church.
I am having a hard time even imagining a situation (as described by the OP) that would actually be licit. But, at the same time, I can’t eliminate the slim possibility. Maybe someone could contrive such a hypothetical.
My point here is that the phrase “it’s valid but illicit” is often used here on CAF; but it’s a mis-used phrase.
There are users who read posts and come away thinking that the word “illicit” means “outside the visible structure of the Catholic Church.” I’m thinking of a very animated exchange I had with one user a few months back who thought exactly that.
So the answer to your question is "yes, it’s illicit."
(with the slight caveat that someone might be able to contrive some additional hypothetical additions that would make the OP’s own hypothetical somehow licit)
But it’s necessary to also explain (especially to the many readers here who might not understand the subtlety) that is is illicit because it specifically violates Canon Law; it is not illicit because it happens outside the visible structure of the Church.