How do I keep my daughter from ruining her life like this?

My daughter is 15 and has been cast as Kim in the nearby professional theater's production of Bye Bye Birdie. The adults leads of the cast are current members of actor' equity. Her father and I are divorced and have shared custody. He is the one who encouraged her to try out for musicals several years ago, something I am quite against. She plans to study Musical Theater in college, and I HATE THIS. I want her to get a REAL major, like education or biology. She needs to stop thinking she is talented enough to succeed with this and face up to real world expectations. Don't tell me that musicals are popular these days - she will simply not be successful with this endeavor. Honestly, why do her father and others keep telling her that this means she could have a future with this? She needs to enter a major where she will be guaranteed a job in the work force.