Grinding sounds in the rear left the side of the vehicle while breaking, some braking sounds worse than others! Btw my Mother has been driving since this issue & was told when she got her tires replaced a couple months ago that her rear breaks would need to be replaced pretty soon.
Well, you've got a professional opinion from someone who has actually looked at it -- they're probably right. They didn't pressure you into doing the work right then and there so at first glance they seem honest. They were giving you a "heads up" and my impression is that they can be trusted.
Brakes aren't terribly expensive and are a normal wear & tear item on any vehicle. If you get to it soon it may only need pads; if its been driven for a while with worn pads they may have started to wear down the rotors or brake linings. That may be the grinding sound you hear, so the needed work may be more than just pads.
Truthfully the rotors on Patriots are rather thin and often they can't be "turned" (ground down till smooth again) so don't be surprised if your mechanic says they must be replaced. If a vehicle isn't used a lot, spends its overnights in damp conditions like parked on dirt or grass, or near the ocean, a river or lake, then its rotors may rust prematurely.
Patriots are relatively light vehicles so the brakes may last a good long time, but sooner or later anything wears out. I got nearly 100,000 miles out of my brakes, but my rear brakes that wore out first. Again, that may be what you are hearing.