Did Pope Francis overstep any official Catholic boundary when he criticized Trump? Pope Francis implicitly criticized Donald Trump as not a Christian when he said, regarding Trump's proposed policy to build a wall between the USA and Mexico and immediately deport all illegal immigrants,:


  "A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian. This is not the [message of the] gospel."
  Pope Francis - CNN - February 18, 2016


Naturally, Trump disagreed with this entirely, saying that "No leader, especially a religious leader, should have the right to question another man's religion or faith" (ibid).

All politics aside and whether Pope Francis' actions lie within some American taboo, did Pope Francis overstep some known Catholic boundary when he chose to enter the fray of politics in the United States by expressing an opinion about the religious faith of a candidate?
There is no doctrine, policy or convention in the Catholic church that prevents a priest, or any church leader, from pronouncing on the rightness of some action or belief.

Historically, over the last thousand years or so, the Pope and other church leaders have pronounced on the policies and conduct of many world leaders. In recent times the Pope has condemned Apartheid, Communism, the Rwandan genocide, restrictions on the Cuban church, and much else.

It has to be pointed out that Pope Francis did not pronounce Trump a non-Christian, but pronounced the actions described to him by a questioner as those of a non-Christian, leaving open at least the possibility that the actions described in the question did not accurately reflect Trump's stance. Trump is also far from the only Presidential candidate both advocating wall-building and claiming to be Christian.