Complex sentence: At his death, the "Science of Triangles" was still unfinished, but, paralleling his own relationship with Copernicus, Rheticus had acquired a student who devoted himself to completing his teacher's work.
Split into simpler sentences: At his death, the "Science of Triangles" was still unfinished. However, paralleling his own relationship with Copernicus, Rheticus had acquired a student who devoted himself to completing his teacher's work.