I had this happen just the other day...I rinsed them in a sink that had been cleaned with a spray containing bleach. That's what I get for having it in the house where other people can find it! I have done the same thing to clothes waiting in a pile by the washing machine. When the hose pumps out the water into the utility sink it can mist, spray or splash. So when I wash something with an oxybleach powder, it can make marks on the waiting clothes. It took me a long time to figure out how it was happening. Check if she might be using a pretreat spray that's staying on too long, maybe it doesn't like the dye in the fabric.