Tipping can be controversial and everyone has their own opinions and practices. That's probably why the client specified that you have to tip the person and what amount.
I don't think you have to tip at places like Five Guys. Isn't that more fast food? I don't think tipping is customary at fast food. I actually haven't been there. There's one nearby though and I might pick up a shop there if it's convenient.
I think the customary 10% for counter service accounts for the fact that the employee is not "waiting on" you as much as when you get table service. But the difference between that and what this client wants is not that much.
I live in a "liberal" state where restaurant servers have to make at least minimum wage -without- tips, and we have a state minimum wage that's above the federal one. In my city, the $15 an hour minimum wage is going into effect in a progressive fashion, to where they get a raise every year until it's up to $15 in a few years. But it's also expensive to live here. People still tip; it's still considered customary to tip and rude not to tip. People tip generously as well. It's unusual for me to be out with friends and see them tip less than 20% for table service at even a decent place, not fine dining but somewhere with table service. I also see people tipping generously for delivery. I personally stick to the 10% for carryout or delivery unless something outrageously awesome takes place. But I would never not tip unless the service was HORRIBLE, because it's customary to tip and that's what all these pay scales and systems are based on.
Some restaurateurs here are trying to change things with the minimum wage going up. They are trying to do away with tipping and instead do a service charge on the bill of about 10% so that the money can be distributed evenly between the front and back of house. A lot of people are saying that tipping is an outdated practice because it places the person's livelihood in the hands of customers, and that it gives preference to the front of house people where the back of house people are making peanuts. Sometimes it even gives power to the front of house people when they are expected to share tips with back of house but they choose not to.
I heard something on NPR where they interviewed some NYC restaurateurs about tipping and changing the practice of tipping. Two things that stood out to me were that people who are predjudiced in one way or another- racist, sexist, etc- are also prejudiced in their tipping, and how tipping started out during slavery and at that time you would never in a million years tip a non-slave person. It would be considered rude TO tip and like you were "demeaning" the non-slave person by comparing them to a slave.