If the customer is operating a franchise of a national brand he really needs to follow the rules of that company's branding guidelines. The brand is property of that national company, not the local customer.
If the customer insists on putting his own creative choices into the sign work go ahead and do the work. But get paid for it promptly, if not up front. And absolutely be sure to save any emails or other correspondence to show you warned the customer. Because they absolutely will try to act like they didn't know any better and make it your fault for not warning them.
I've been in this business long enough to see franchise operators get their @$$es in a sling with a national company's home office for doing stupid, not-compliant things with their brand. They insist on changing colors, fonts, using an out-of-date logo, etc. A regional or national manager sees the work and gets angry. The local guy is ordered to replace the sign, often out of his own pocket.
Trademark infringement is another big one. I've seen local, small businesses get sued for creating logos too close to a national brand. They say "no one's ever gonna notice, we're in a small city, no one's gonna care." Uh, wrong. Technology is a heck of a thing. The local business can get busted via a simple Google image search or by a Google car driving by their business and putting the Street View imagery into Google Maps/Earth. Any social media outlet can get them busted too.