Your friend suffers from binary thinking. He can become a master at ReactJs and be a well rounded developer who contributes across technologies. What if he were determined to become the best Basic developer? Where would he get a job today?

I'd also offer that working in other technologies will give him a perspective on ReactJs, it's strengths and weaknesses, thereby making him a true master of a technology. If all you see in front of you is what you are familiar with, you will never encounter or understand the limitations of where you are.