Treason in the USA is narrowly defined in Article Three of the Constitution. The person in question has to be actively helping a wartime enemy, and there have to be two witnesses to the acts in question. Neither is really the case here.
This incident is really something that falls more under the 1798 Logan Act which makes it illegal to negotiate with foreign governments having a dispute with the United States to try to undermine the US Government position.
The problem with the Logan Act is it historically has never actually been used. The closest it came was a couple of times in the 1800's when a couple people in different incidents got indicted but were never prosecuted. There have been numerous incidents since then equally worthy. In recent times it was threatened with use during the Nixon, Ford, Regan, Bush II, and Obama administrations.*
There's also an argument that the Logan act is unconstitutional, so if it were to be used, it would likely have to go through a Supreme Court challenge.
* - There are persistent claims that the Reagan campaign interfered with the negotiations to end the Iran Hostage Crisis, which would theoretically put Carter's administration on this list too.