This issue is controlled by 1st Amendment case law, and it is frequently a "sea of storms."
What the law boils down to is whether or not an artist's work is "transformative." The "inquiry is whether the celebrity likeness is one of the `raw materials' from which an original work is synthesized, or whether the depiction or imitation of the celebrity is the very sum and substance of the work in question." (Comedy III Productions, Inc. v. Gary Saderup, Inc. (2001) 25 Cal.4th 387 406
.) If the "product containing the celebrity's likeness is so transformed that it has become primarily the defendant's own expression" of what he or she is trying to create or portray, rather than the celebrity's likeness, it is protected. (Id.
at pp. 406-407.)
Celebrities rarely sue over straightforward portraits, because they like the publicity .In fact they are more likely to sue over distortions of their likeness which holds them in a negative light. Ironically, the celebrity's case is far
better where a portrait is simply a photo-realistic rendition of an artist, than it is where the celebrity is savaged by the artist's rendition -- because the act of disparagement, parody, editorial commentary, etc., is precisely what the 1st Amendment protects -- i.e., dissenting opinions.
From a pure legal view, if your artist is creating photo-realistic artwork, he/she could be sued. The more independently "creative" or opinionated the rendition, the more protection is afforded the artist.
That's how the law in this area is interpreted. Hope this helps.