Somewhere he and your daughter want to live, and opportunity to specialize...I chose mine for that combo of reasons (state school where I got residency after a year, life outside school was great for my wife with a good support system and lots for our family to do, and very strong specialty program in what I wanted there which allowed me to get into the specialty I wanted where I wanted).
3) Apply early (be ready to go when AADSAS opens up) to 7-10 schools if he's a competitive applicant (DAT above 20, BCPM GPA over 3.7). He should know a little about each school besides just location or name so he can tell the school in a interview why he is attracted by their program. Applying early May give him the opportunity to plan interview invites at multiple schools in one trip (for example, I hit UNC and VCU in back to back days as they are only 2 hrs apart).
4) Have good letters of recommendation. I cannot overstate this. I spent 4 years as a dental school AdCom member and the LoRs we're a big part of deciding who - outside of the DAT/GPA all-stars - got interviews. Most schools have some sort of matrix where the top applicants by scores get auto interviews; the bottom get passed on; and the middle chunk have say half as many interview spots as people. So you want something that's distinguishes you an good letters that can talk about specific aspects of you as a candidate do that.
5) Find out what the real cost of attendance is. What the school publishes or financial aid covers is one thing; what current students say it actually costs is an entirely different thing. A school may say it will loan $35k/year for tuition plus $20k room and board, but really they may not loan enough to cover actual living costs and you have to come up with another $15k/year and before you know it 4 years later you're down $300k+. Or you may go somewhere where tuition is half that if you get residency and it's inexpensive to live (like we did) where the $15k/year loaned for room and board was enough for me and my wife and kids (though we ate a lot of beans and rice, and skipped vacations and eating out and entertainment and stuff) and get out of school with $130k in debt. So basically find out what the school says about $$ then ask those currently there.

And a few other non-application but worthwhile nonetheless points:

-Encourage him and your daughter to be open to settle near where they go to school if they like it (assuming they are from a place saturated with dentists). We did that and we've been in the South ever since. Best move we could've made.

-Do your SIL a favor and prep your daughter now if she hasn't already started for living poor while in school and immediately after. I know lots if guys whose wives wanted to spend like they were married to a dentist before, during and after school when if they were smart they'd have been saving and knocking off student loans.