When I was very young and not yet frum I worked for a summer in a local site of a non-kosher restaurant chain. You don’t want to know what went on in the kitchen. Just thinking about it still makes me shudder. If you think the help actually keep the rules that the management sets out for things like cleanliness, putting the dishes the right way in the dishwasher, etc., I’ve got a nice bridge to sell you. The chain doesn’t pay the help well enough to make them care about doing a good job, and half the time they don’t have what they need to do it, anyway. A barrista doesn’t make bubkes, and the turnover rate is astronomical. And there’s no friendly mashgiach dropping in randomly to check up.
Buy a blender and learn to make your own frappucinos. It’s probably cheaper and a good deal safer for both your kashrus and your physical health!