Well then tough **** for him. Two other countries are allowed to engage in whatever discussions they want without his sayso. They are allowed to ask a private citizen with experience in the field for help. He was not doing it as any specific member of a US delegation and was not interfering in the US's actions in the matter.
This, of course, highlights why the right's chest-pounding about withdrawing from the deal is so stupid. While we are probably the most important member of the deal in terms of economic size, it is a multi-party deal, not a two-party deal, and thus we don't hold the cards we would in the former. The sanctions on Iran before the deal worked so well because we had the cooperation of the major European states; us or them alone would not have had the power to bring Iran to the table and get the concessions they did. Now us backing out of the deal and trying to re-impose them will not have the same effect because, get this, other countries are other countries and might not automatically do what we want, despite how much Trump thinks he such a super-awesome negotiator, and without them our actions are largely equivalent to a child yelling into the void.
