Should prove interesting for Kerry. Kind of hard to guarantee any US commitments when you have zero influence on the current administration. More likely he'll get held hostage or beheaded.
The Logan Act (1 Stat. 613, 18 U.S.C. § 953, enacted January 30, 1799) is a United States federal law that criminalizes negotiation by unauthorized persons with foreign governments having a dispute with the United States.
It is difficult to prove. In this instance the idiot is publicly admitting it in a bool and in public interviews. Pretty easy case, unless Trump is using Kerry as a pawn.