And not just sola fide is missing here; so too is the Reformed view of the perseverance of the saints.
Look to yourselves, that you may not lose what you have worked for, but may win a full reward. [2 Jn 8]
In the first place, St. John says that there is something that his readers have worked for: this is completely inconsistent with the Protestant idea that we are saved by faith alone.
In the second place, St. John warns his readers against losing what they have worked for. But this strongly suggests that one could lose his salvation, which is contrary to the Reformed notion of perseverance.
In the third place, St. John speaks of a reward that is based upon what his readers have worked for. But this contradicts every Protestant claim about our works being rewarded (generally they say that our deeds are not rewarded), and particularly the claims of the Reformed, who say that every tiny little sin deserves hellfire, that no one is free of tiny little sins (or even big sins), and so it is impossible on the Reformed view for anyone to gain any reward by their works.
In short: 2 John does not support Protestantism.