She is a 4th year PhD student in applied mathematics working with an established adviser who has plenty of grant money. She's just doing the standard thing... I see the same situation all over my own university's science departments.

PPO can be used at any provider and typically only get you a slight discount for any service beyond a basic cleaning. As a grad student, I've had to get a crown and 3 fillings (arising as complications from wisdom teeth, which I had removed while a full-time engineer prior to grad school). I realize my expense is at the far end of the distribution, but that's why we have insurance. Grad students (like me) aren't in a position to handle these expenses and our insurance is not remotely adequate. I had to go into significant debt to finance my own dental work.