Do you continue to encourage that person, or do you try to do that carefully while injecting a dose of reality?
Here's what I've written elsewhere on this matter, and I'll copy it here verbatim, with added commentary afterward:
I cringe every time I read in here of kids still in high school, or barely starting college, who already either are focused on a particular career, or already made up their minds that on a particular, exact career that they want to do. Now don't get me wrong, there's nothing wrong with having an ambition and aiming to want to be something. However, one needs to step back a bit and figure out if the "choice" being made here was made based on having all the necessary information (i.e. a well-informed decision), or made entirely based on superficial perception.
There are two important issues here that should be addressed and considered.
(i) It is highly unlikely that an 18-year old knows extremely well what is involved in being, say, a theoretical astrophysicist. So how did someone like that arrived at the conclusion that that is what he/she wants to be? More often than not, this person saw some TV shows, or went to some facility, or read some news coverage, and over a period of time, "fell in love" with the idea of being a theoretical astrophysicist.
(ii) It is also very likely that this person hasn't yet been exposed to ALL (or at least, a lot) of the exciting aspects of other field of studies. It is one thing to have seen all the "merchandise" and then make an informed selection, it is another to have only seen one or two and decided that those are sufficient to make a choice.
While there is nothing wrong with having a goal, there is a lot of things wrong when such a decision causes one to have blinders on and not even consider looking at other possibility. It is one of the reason why I conducted a non-scientific career poll on the PhysicsForums. I wanted to see how many here who actually ended up in the VERY exact field that he/she envisioned when he/she was that young. If you simply look at the results, you'll see that only 15% of the poll participants ended up in the very exact career that they envisioned[*]! Significantly more of the participants end up doing roughly the same type of field of study, but not exactly the area of specialization that they had in mind.
What is the lesson in all of this? The lesson here is that, if you're just starting out in your academic life, there's a VERY good chance that you WILL NOT end up in the very exact specialization that you had in mind. That is a very important take-home message, and could be one of your first smack of reality. What this means is that you should NOT close the door on other subject areas just because you already have an ambition to be something. Just because you want to be a theoretical astrophysicist doesn't mean that you shouldn't at least look into solid state physics or read new discoveries coming out of atomic/molecular physics. There's a good chance that you will not be a theoretical astrophysicist, and you need to prepare yourself for such a possibility. It is why I've always tried to emphasize an undergraduate education that is as WIDE-RANGING as possible. Want to be a theorist? Well, take that extra lab class anyway! You'll never now that your ability to make that thin-film deposition might be the very skill that get you that job, or that graduate school admission. Idealism can only go so far before financial reality steps in and smack you on your face.
[*] I am still skeptical of this number, and so far, only one participants have given an explanation on his selection. I think this number might be even significantly lower than what we end up with. I am guessing that many didn't actually read the full options posted in the first message of the poll. Of all the physicists that I've chatted with, I don't ever remember even one of them telling me that they are doing what they had in mind exactly when they were 17/18 years of age.
The fine line that I would walk here is to not discourage someone that young so that he/she continues to nurture that interest in physics. But the reality is that, there is a strong possibility that he/she would NOT go into such fields. Physics is such a wide subject area, and the less glamorous ones are often the ones that (i) have a larger chance of employment (ii) has more funding (iii) wider and larger number of opportunities. In many of my contacts with high school students, I think when the opportunity arise, I try to convey this view that physics is more than just the LHC or string theory. It is also the iPod and the silicon chips and the laser and the MRI. I can only hope that somehow, those blinders would start falling and they could see a wider horizon.