Your brother could contact a local television or radio news dept. with the details. The restaurant might change its policy under pressure of negative publicity.
Another approach: your brother and his friends may want to consider going to the restaurant with a formal complaint.
They could directly send a letter of complaint with details of the incident and specically identify what they want the restaurant to do (e.g., change policy, apologize, etc.). They could send it by registered or certified letters to the owner and manager.
They could contact a DC attorney (maybe military legal aid could direct them to an attorney who would do this free?). The attorney could help with the letter of complaint or just get involved if the restaurant won't reverse course, apologize, and/or pay damages (if the Marines want some sort of damages).
Possibly they could ultimately file a complaint (click here) with the DC Office of Human Rights.
Per the linked web site, discrimination is not allowed on the basis of "source of income." I could be wrong, but I think that would outlaw discrimination based on someone's occupation as a Marine.
Thanks for the great suggestions! They are in the field until Saturday night but I'll pass along the info. I know there was some idle talk about vigilante-ism, but they're just not the Charles Bronson type.