To Whom It May Concern,
or
Dental School Admissions Committee,
I am writing to (insert adverb) recommend (insert name) for admission to dental school. (State qualifications here, how well you know the applicant, why the adcoms should listen to what you have to say)... maybe something like this: "I have been a practicing dentist for X years and can confidently attest to (name)'s work ethic, character, and academic aptitude. In my X years of practice, I have not met someone more dedicated, personable, and prepared for a career in dentistry."
This is the dentist's opportunity to substantiate the accolades they gave you in the first paragraph. Here in the second paragraph maybe give a few examples of what you did, how you helped out in the office, your patient interactions, etc.
In the final paragraph, reiterate your purpose and restate what the letter is all about.
Sincerely,
free99, DDS