I had an "oppositional" son--first born.  He was that way until he left home.  Here are the ways I worked with him:

1.  You have a choice--a.) get a dinosaur sticker to put on your sticker chart for going potty or b.) staying in the bathroom until you have done it-- without any sticker.  (When he filled the chart, he got a reward.)

2.  Then I counted to three--not slowly, but at a normal 1-2-3.  If he let me go to "three"--he was placed in the bathroom (or "think it over chair" for other things).  If he couldn't stay there by himself, I physically held him. Sometime we were in a wrestling hold for 1/2 hour.  I always made sure I "won"  the control game.

I held to this method and got him potty trained and trained in a lot of other things, too.  What you're wife is dealing with is an "oppositional" child.  Google it.  It can be a symptom of things like ADHD or it can stand on it's own.  My son was just "oppositional". He wanted to be in control and make the rules.  This is not possible for a 4 year old boy--nor a teenager.

In the end, my son is an adult now and we have a great relationship.  He grew up respecting me and appreciating what Mom wouldn't let him get by with.

BTW: my son also tested teachers throughout his school years.  He got a long great with strong male and female types, but if anyone was a wimp, look out! One thing he knew with the wimpy teachers, though--if he went too far, he had to face MOM!

Be strong & consistent.  It will make a world of difference.

Mrs. Fefernuse

P.S. If you, the father are around, does he get to watch you do it?  That's also helpful.