I think - it would be nearly impossible to teach 16 year old the difference between passion and practicality without causing him/her to entirely lose interest in the process. Most 16 year olds simply don't have enough life experience and maturity to consider all the implications. In some ways, at their age they shouldn't be all that concerned with all the nouances of "maintaining the right balance" in their lives.
I *think* the best advise anyone can give is to direct them in such a way that knowledge and skills they gain are usable in more ways than just one. Skills to observe, skills to be patient, skills to think independently, not to mention skills to write, read, listen, etc, etc, etc are invaluable. Maybe encourage her to pursue this goal.... then along the way, steer her into directions that can be useful to this goal as well as others so she can fall back on it if she has to.
Yeah, maybe her interest will change too. I know mine did.