There is quite a bit of information left out of this question so it is hard to say for sure what exactly transpired to prompt the officer making the arrest in the first place and why one party was taken in while the other wasn't.
The Criminal charge is Assault 4 but there may be factors that influenced the officer's decision, including if the act was done in front of the officer, if the officer believed that the offender was out of control or posed a danger to the other party or to other persons on the premises, if the offender was someone who had a history of acting out in such a manner as to prompt concern. Were there children who witnessed the biting? That is something also that comes into play and may have influenced the police officer's mind or decision to act.
Officers may be told not to distinguish between genders and physical traits when coming to view people but they are human and carry their own notions. If things are as you say, there must have been something that prompted the officer to take the 5'3" woman into custody and not the 6'3" man. Was the man's wounds noticable or readily apparent? Was the woman on drugs or acting in a manner that was just so outside the norm that the officer may have suspected something? There is a lot of information that would go much further in getting you a more concrete opinion than what you posted above in your question. I would highly suggest that she find an attorney who handles criminal work but also has some background in domestic violence situations.
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