My underage friend got drunk and cops sent him to a hospital. Used fake id there. Can they bill him?

If he gave false info (which included a fake address?) then how would they know to bill him? How would his insurance coverage know it was him under a fake name? By trying to correct this, he will be exposing himself to discovery of the fraudulent actions of providing them with the false ID. By leaving it uncorrected, he is in essence comitting theft of services. In regards to him using fake ID, yes there are criminal charges that can be filed. I suggest that he discuss this entire situation with an experienced criminal defense lawyer admitted to practice law in DC as soon as possible. Until then he should exercise his right to remain silent if the cops come calling. Good luck.

DISCLAIMER
This response does not constitute legal advice. Given the nature of this website, it does not create an attorney-client relationship. This answer is provided solely for informational purposes, for you to use as a starting point when speaking directly with a lawyer in your State. I urge you to immediately contact an experienced criminal defense lawyer admitted to practice law in your State before you make any decisions about this case.

If he gets away with it, fine. If he messed up somehow and gets caught using fake ID then he will have to deal with the legal consequences.