How does the process of being arrested and charged for criminal assault work? My friend was just arrested for assault.

When a person is arrested for criminal assault, the arresting officer will file an arrest report.  It is then up to the prosecutor to decide whether or not to file charges.  If he decides to file charges based on the assault damages, he will determine whether or not it should be charged as a felony or a misdemeanor, and he will then prepare an assault complaint.  If the case is charged as a felony, it will be brought before a grand jury, who will decide what charges, if any, to file.  They may decide that the case will not be pursued.  If your friend is in custody, criminal law typically provides that an assault complaint must be filed within 72 hours, and some jurisdictions require that charges must be filed even sooner, within 48 hours in California, for example.  I urge you to encourage your friend to get advice from a criminal defense lawyer who can explain assault damages and the process of filing an assault complaint to him, and provide him with representation.