Not necessarily, one could argue that he was giving "aid" to an enemy by giving them speaking power over a large audience, but I'm no lawyer.

There is an argument that it is unconstitutional based on US vs.Curtiss-Wright. Any Supreme Court case would look to that, and likely support the President.

Wiki excerpt, emphasis mine:

Justice Sutherland wrote in his opinion of the Court:

The ["powers of the federal government in respect of foreign or external affairs and those in respect of domestic or internal affairs"] are different, both in respect of their origin and their nature. The broad statement that the federal government can exercise no powers except those specifically enumerated in the Constitution, and such implied powers as are necessary and proper to carry into effect the enumerated powers, is categorically true only in respect of our internal affairs.

And he added:

It is important to bear in mind that we are here dealing not alone with an authority vested in the President by an exertion of legislative power, but with such an authority plus the very delicate, plenary and exclusive power of the President as the sole organ of the federal government in the field of international relations–a power which does not require as a basis for its exercise an act of Congress, but which, of course, like every other governmental power, must be exercised in subordination to the applicable provisions of the Constitution.

While the Constitution does not explicitly say that all ability to conduct foreign policy is vested in the President, it is nonetheless given implicitly and by the fact that the executive, by its very nature, is empowered to conduct foreign affairs in a way that Congress cannot and should not. The Court stated that "there is sufficient warrant for the broad discretion vested in the President to determine whether the enforcement of the statute will have a beneficial effect upon the reestablishment of peace in the affected countries."