The Simplex Prestige shifts just fine as long as there is no more than a 10t difference between the chainrings. I doubt that he would notice much difference if he switched to modern derailer on his UO-8.

The Gitane Tour de France and Peugeot PX10 above both came with plastic Simplex derailers that shifted very well until the plastic parts got brittle and broke.

Front derailers haven't vastly improved over the last 30 years like rear derailers have.