In year 10, I would just be getting them to read and find out more about the profession so they have a fuller understanding of the job. What has sparked and motivated this interest for example- a nice family dentist? friends/their parents? teacher remark? social media etc? Do they like working with their hands? Are they interested in people in general?
GCSEs grades are important in that certain dental schools require specific minimum grades eg 7, 8s in English , Maths etc. Read the Dental Schools Council booklet for latest criteria and to ensure she eventually chooses suitable A levels.
She really doesn't need to 'work on her CV' at this stage. And although dental work experience is certainly desirable, several dental schools have dropped this requirement as they recognised the barrier that this can create for those without relevant connections, social capital or just plain wherewithal.
I'm not sure that drop out rates are actually 'significant' as another poster claims (this can be checked in Higher Ed data) and we all have different peer groups, but there certainly are some people who discover they are unsuited, or cannot keep up.
I'm a dentist coming out the other end of my career. It's been good to me. The pay is good and generally secure. I've enjoyed helping people be and feel healthier and have better oral health, solving pain, working with great colleagues, watching my patients grow up, among many other things. The shopkeeper, excessive admin and unrealistic patient/ workplace expectations parts not so much! I'm not sure I'd agree with the comments on dedication (my family is my first priority), but I am disciplined, interested in keeping up to date and bettering my skills and knowledge, and I do like to be busy and work hard. Lots of jobs are stressful, but it certainly is high pressure and responsibility. You can learn to manage that, but I agree it's not a job for people who don't cope with stress. BTW, most of my cohort are still working and all but 2 are is still in dentistry (one 'married well' -another dentist, as is common- and never worked after kids, the other did Medicine and works in MaxFax)
So good luck to her. I'll check back periodically in case I can offer any more insights.