Without context, working for free requires careful thought. But in the context of this conversation "free" means without monetary compensation, but you're still receiving the benefit of the mentorship (and thus not free).
TLDR; No one's advocating working for free. The Warren Buffett approach is "trade money for time with a good mentor."
TLDR; No one's advocating working for free. The Warren Buffett approach is "trade money for time with a good mentor."