Volunteering as a dental assistant can help her look really good as an applicant. If there's community clinic that does pro bono dentistry for the poor, sometimes they'll let college students volunteer as dental assistants. Getting involved in her college's pre-dental club can also give her great dental-related service opportunities. She has to show the schools that she's serious about dentistry.
I agree with the above statement about needing to increase her shadowing hours. Shadowing is so boring compared to actually doing dentistry, so I feel her pain. haha
Also, she needs to make sure her application is awesome with a great cover letter and solid letters of recommendation (preferably from professors she got A's from). The volunteer work she has already done is great, and she should definitely put that on her application as well.
Crushing the DAT should be her number one priority. Chad's videos on Coursesaver.com will help her with chemistry more than anything out there. Also, practice tests are great. I really liked DATBootcamp.com. She should use the DAT as an opportunity to prove that she can do well in dental school.
Apply broadly, like you said. It's expensive, but I would at least apply before jumping into a year of post-bacc -- especially if she does really well on the DAT and the rest of her application is excellent.
And if it doesn't work out, a post-bacc with continued dental-related volunteerism is a good way to go.