Welcome to the forums!
I assume this is for a federally-backed loan? Or was it private? If private, was there a judgment already?
Disclaimer: I know very little about SLs. I'll defer to others. However, I'm an employer and not too long ago had a CA collect on behalf of a federally-backed loan for an ex-employee. I enjoy toying with CAs and never revealed that the employee was fired a year prior. He really did reveal too much info about the debt, but he wouldn't back down and then in his anger, he said that he would garnish our ex-employee's wages from our payroll account. I told him he can have our accounts garnished if he wants, but that he better get a judgment from the court first and have garnishment papers served. He said it wasn't necessary and that he can take whatever money he wanted at any time to cover the unpaid loan. We parted company but the conversation bothered me. I later learned that he was mostly right. A judgment wasn't needed and they can seize our payroll to cover this guy's loan. Now maybe there's some sort of due process involved, I dunno, but a judgment is not needed by the courts on a fed-backed loan.
In your case, it sounds like the same tactics used on me were used by your HR person. That could explain the heat. Per the garnishment limits, you'd have to do some searching. I bet it's written out there somewhere what the limits are. Where I live, 25% is standard for garnishments after a judgment. The court gives us a paper to follow and there are tiers with 25% being the top tier for net incomes (after tax or court-ordered obligations like child support). That 25% is state-law and certainly isn't related to your HR's 25% request. I'd look up the law. If the law did show 15% you can always re-approach your HR folks and have them back down to the 15%.
BTW, they can't ignore an order. In my early days at the company, I had an employee who spent $$$$$$$ on worthless furniture from one of these rip-off places and financed it too at a high rate. She paid what she thought she owed and then defaulted. A judgment was obtained against her and then we received a garnishment summons. At the time she was going through a foreclosure, and had some medical issues with her kids. I foolishly ignored the summons to try to help her buy some time. Sure enough, the full amount of the judgment came out of our payroll account a couple of months later. That's the last time we ever did that.
Current Score: Eq: 823 Tu: 830