First, an important clarification. Federal parole was eliminated for crimes committed after November 1, 1987 under the Sentencing Reform Act of 1984. What the federal system uses today is called supervised release, which functions similarly to parole but operates under different rules and with different consequences for violations.
If your husband has a federal hold on him, it almost certainly means he was on federal supervised release when his state offense occurred. Committing a new crime while on supervised release is one of the most serious violations possible, and the federal consequences stack on top of whatever the state is doing, not instead of it.
Here is how that plays out practically. Once your husband completes his state sentence, the federal detainer kicks in and he transfers to federal custody. At that point a federal judge holds a supervised release violation hearing. The judge can impose any amount of time up to the original federal sentence that was suspended when supervised release was granted. The guidelines for supervised release violations are calculated based on the grade of the violation and his criminal history category, but the judge has significant discretion.
A new criminal conviction while on supervised release is a Grade A violation, the most serious category. That typically results in a substantial period of incarceration, potentially years depending on the original sentence and the circumstances of the new offense.
He needs a federal criminal defense attorney involved immediately if one is not already in place. The federal violation hearing is separate from the state proceedings and requires its own legal representation.