Based on Keynes theory of economics, this statement would have been false. Keynes would not have been a good tipper.
Keynes economics was based on creating rigid salaries and that a dramatic increase in salaries either way could affect the economy. Keynes believed that the greatest change occurred from output, not by increasing money into the economy through increase of wages. He further believed that the economy existed in the short term.
Most importantly, Keynes believed that there was a limited range of increase in wages. By giving an excellent tip, he would have greatly increased the range of that wage. The wage then would have exceeded the standard, and possibly led to unemployment or inflation.