If we are speaking of somebody in the custody of the Department of Corrections and this is a criminal warrant that could result in another prison term, he should take advantage of the 180 day rule.

If this is a misdemeanor warrant to which the 180 day rule does not apply, he should consider petitioning for habeas corpus so that he can get before the court and address the warrant, and ideally get credit for time served.

If this is a civil warrant for nonpayment of a traffic ticket, then he should consider his options to resolve the warrant -- such as by finding a way to pay the warrant.

Without knowing more about the warrant, we cannot comment on how it might affect halfway house eligibility; but it is reasonable to say that if placed in a halfway house he's going to be expected to resolve the warrant.