Complex sentence: Another author is free to describe the same theory in his or her own words without violating copyright law, and in fact need not even give credit to the original author (although failing to do so may be considered plagiarism, an ethical transgression).
Split into simpler sentences: Another author is free to describe the same theory in her own words without infringing on the original author's copyright. In fact, the second author does not even need to credit the original author (although failing to credit the source of an idea may be plagiarism, an ethical transgression).