The immigration will most likely move him to an immigration detention facility in WA. Once there he will be served with charging documents listing the reasons why he is being held. Once served, he will be taken to an immigration judge to answer those charges against him. I would advise you to contact an immigration attorney in the area so he may have adequate representation. Your employee may be eligible for some form of relief (possibility of remaining in the U.S.) that he may be unaware of.
Disclaimer: This answer does not constitute legal advice nor does it create a attorney/client relationship.
The best thing that you can do is to contact an immigration attorney to represent your husband's employee. DUI, in and of itself, is not a deportable offense. It is however punishable by enough potential jail time that it could fall under the INS's grounds for exclusion. It sounds like the DUI conviction is only part of the problem. Seek the counsel of an immigration attorney immediately.