My opinionated opinion: It's NEVER okay to tell someone not to follow their dreams. It's okay to say: Get a job to support yourself WHILE following your dream. Be informed about the market. Learn the craft. KEEP following your dream.
Giving up on dreams... bad. I gave up on my dream of being a Broadway actress because I realized I didn't have the chops to make it to that higher tier. That was a sad year. I had the writing, which was my first dream, and I returned to it. Things got better. I shudder to think what kind of embittered 9-to-5 angry grunt worker I'd be today without a dream.