What's unlawful around working two jobs? Happens at high end (consulting CTO John Carmack, or Twitter & Square CEO Jack Dorsey, and in the US happens significantly often at the lowest end (retail workers in two jobs).

It's unethical in that the two employers are unaware of that fact, and he's advising deliberately not doing work in bad faith (whereas the boards of Meta, Twitter and Square were aware of the previous situations, and your retail shift manager knows he's not scheduling and paying a living wage), but illegal probably not.