Could I face criminal prosecution for housing someone that I just discovered has a bench warrant what should I do

While theoretically you COULD be charged with harboring, the chances of that happening are remote at best. Depending on the State where the warrant is from, chances are that state is not actively looking for him/her. And even if they are, they are usually just happy to get the person in custody. Unless you ADMIT to the authorities that you knew that the person had an outstanding warrant, no one is going to bother you. So don't admit to anyone wearing a badge that you know that.

If this person is stopped and a background check is done here in California and the warrant is discovered, you would likely not be implicated and thus have nothing to worry about.