His bes bet is to contact an experienced criminal traffic defense attorney in his area and set up at least a phone meeting. Usually cases of this nature can be resolved relatively easily without a lot of trouble and heartache. But the facts of the case and your cousin's situation will determine the specifics for him. He should consult with an attorney.
Prosecution practices vary from county to county, and upon information and belief the DA in Waukesha has some very specific policies in this regard. The facts and circumstances of this arrest are likely to be very important in how the prosecution treats this new offense. Help him retain experienced local counsel, and have him take care of any unfinished business from the underlying OWI case, like assessment compliance, treatment, occupational licensing, and fine payment. A person who is doing his or her best to comply with the court's orders in the earlier case will come out better than one who is perceived as thumbing his nose at those orders.
This answer is provided for general information only. No legal advice can be given without a consult as to the specifics of the case.
If your cousin was just outside his hours on a valid occupational, he has a shot of getting this reduced. In front of most judges in Waukesha, he is unlikley to go to jail, but he needs to consult with an attorney before his first court date. Just a free consultation might be all the help he needs to avoid jail and a criminal conviction.