Does their application have a “Why College X?” essay? As @calmom said, be sure she knocks that out of the park. She should be specific and enthusiastic. Not “I like small class sizes”, but “I like that the college offers X major with the X subspecialty.” Discuss majors/minors, facilities, any profs she met or classes she sat in on, any ECs specific to the college, etc. And let her enthusiasm show in the essay. The “Why X” essay isn’t a time to be creative or play it cool. A genuine, well researched essay that shows why the school is a fit for her and she is a fit for the school can go a long way. But I do agree that saying it is her number one choice could hurt you in the merit hunt.
Of course, as I’m sure you know by now from reading the threads out here, part of your job is to help her find multiple schools she could be happy with if accepted. Don’t let her get set on one school – one way or another, that often leads to heartbreak.
If you tell us the school, we might be able to suggest similar schools to look at so she has more than one school that she loves.
Also – her favorite school now might not be her favorite by next May. My D2 took a class senior year of HS that changed her ideas about her major (and she is now a PhD student in that new direction she took). Size and distance from home preferences can also change. Finally, there is a lot to be said for collecting all the acceptances, reviewing the FA/merit offers, and then attending accepted student visits at the top 2-3 choices. 24 hours on campus is very revealing, and can definitely change your mind. My D2 went into her top 3 accepted visits with a pretty specific preference order. It totally flipped after the visit – the 3rd place school won out, and was a great fit for her for 4 years.