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I'm sorry to hear about your father. You are correct that inmates are entitled to adequate medical care. And while every jail has its own regulations, allowing another inmate to attack another is not allowed and is probably negligence. As to how to proceed, it is up to you. You can either try to find a lawyer in your area who handles these types of cases (many criminal attorneys also handle civil rights issues that arise out of criminal cases) or you can see if your local ACLU chapter would be willing to help. The ACLU frequently litigates cases relating to prisoner's rights, including those cases where inmates are denied proper medical care, and so it is often a good place to start to see if they will assist you.
Here is their page on prisoner's rights and medical care that shows several of the cases they have handled on the issue: