Reductive things oversimplify information or leave out important details. A reductive argument won't win a debate, because it tries to make a complex issue much too simple.
Your friend may recommend reading the "CliffsNotes" version of "Moby Dick," instead of the novel itself — but if you do, you'll only get a reductive summary of the plot and themes, instead of a long, nuanced book. Reductive shares a root with reduce, or "make smaller," the Latin reducere, "bring back." The earliest meaning of reductive was "that brings back."