He might be right. I started playing offhand lefty tennis, for the fun of it (to feel like a beginner again and also to be able to play when there are no 4.5s around) and you would be surrpised how quickly you get the knack of it. Even with serving. And my one hand righty backhand seems to have gotten better since adding playing lefty to my practice.
Facing Federer book was called "pretty addictive" by Steve Tignor :)