Commitment laws very by state, however, most states have a maximum hold of 72 hrs. for observation and determination if the patient is a danger to himself or others. If he has no criminal charges pending, and is not a danger to himself or others, he should be able to check himself out. While doctors can strongly advise a patient to remain admitted, they can not force him to do so. Your question does not give enough information. Based on the fact that he has been held for two weeks, I suspect he may be under arrest for the brother's "complaint." It would be helpful to know what the "complaint" was, and what the doctors' determination is.
This appears to be an Involuntary Commitment which is the practice of placing a person to a psychiatric hospital or ward against his will, in compliance with your state’s mental health laws. Commitment is normally time-limited and requires reevaluation at fixed intervals. Generally when the police are called and arrive at the scene, they assess the situation from witnesses and other surrounding facts to determine if the patient will be a threat to himself or to others or both. After determining the patient is suicidal or a threat, then the patient will be committed at a mental hospital (involuntarily). Thereafter, the patient is evaluated at fixed time intervals under close observation. Sometimes stabilization is by ways of medication or psychiatric therapy or both. However, if psychiatric personnel do not believe that the patient can be safely discharged or released, then the patient will be held for further evaluation. This is probably the reason your friend has not been discharged. Once again, the laws of your state may differ from the above. Without knowing more facts, it is best that you contact an attorney in your state directly for advice.
ATTORNEY DISCLAIMER TO THE ABOVE ANSWER: The information that I have provided to you is for general information purposes only and shall not be construed as legal advice for your individual circumstance. This information is not intended to create an attorney-client relationship and you should consult with an attorney in your State directly so that the attorney has the full facts, documents, and an account from other witnesses.