First, Government has the legal right to garnish for a student loan debt. That's how students can get student loans by the guarantee of the government that they will get their money back.
Secondly, I have had to pay child support myself for my two daughters, but now they have recently moved back in with me since end of December 2006. My child support is still coming out of my check, because it takes a while for the AG & the attorneys to file everything appropriately and get it all set up. My ex-husband, when we did all this re-arranging of custody, had agreed that he would return the money back to me. It was looking like he wasn't going to for several weeks but then finally, last weekend, he gave me two checks. One for my child support being returned, and the other for his child support. WHOOYOO!! It was like a savings account I didn't think I had!!! hahaha...
Anyway, to stop his CS garnishment, he will need to do it with the AG or get an attorney himself. With the AG, he will have to prove how long the kids have been in his home, unless him and his ex have a signed agreement with the date. Then they can back-charge her for his payments AND charge her for child support PLUS interest -- because it was kind of a "loan" from you... And it's pretty high interest, I believe like 16%...
Back to the student loan... he defaulted on the loan otherwise, they wouldn't garnish his checks. What he can do is apply to consolidate the loans to lower the payments and get it out of the Governments hands.
Best of luck to you!