I submitted a draft of my personal statement along with a resume and a post-graduation update (I'd been doing informal post-bacc). I think if the writer sees how you're presenting yourself to dental schools, it could help them tailor their letter and in the end, make for a more cohesive application.
In terms of the key points, I'd consider what your relationship is like. If you did something unique with them (e.g., research, served on a committee with them, etc.), definitely request that they emphasize that experience. If not, figure out what you want the dental school to know about you as a person. You're actually lucky your professor asked for direction because it puts you in the driver's seat, at least to some degree.