I'm not in the crim part of my review yet, but from what I remember the cops don't need a warrant to arrest someone, all they need is probable cause.

The best advice I can give to anyone who gets arrested in a shady situation with the cops is simply to obey their commands (even if you know they're wrong) and don't say a fricking word (other than to say you are employing your right to remain silent, and that you'd like to speak to an attorney before talking to them).

Your friend also has a right to know exactly what he is being charged with - cuz although possession of controlled substances is possibly an offence in your state, having a party is not.