Either way, whether she was a non-rev or not is not relevant. What is important is the bag turning. Ultimately, as long as the bag meets the dimensions of United carry-on, you should not have to check it. You fly over 50,000 miles on United airlines, giving them money, loyalty, and business, and in return, they give you the privelege of boarding first so that you can put your bag in length-wise.

If this bag does not fit the United regulations, she may have a point; you may have to check it in because it's simply too big and should not be there. However, if it meets regulations, and you got on first, she should be the one who needs to gate-check her items, not you. That's part of the deal that United gives you in exchange for your loytalty of flying 50,000 miles per year.