Dangerous only pertains to how and why you're conducting a business such as you are doing in this instance.
If you're a seasoned professional and have all kinds of insurance... don't you know or understand the ramifications if something happens to you while you're on this job ??
Should something go wrong and you break a leg or God forbid... something worse.... the municipality, his insurance and your insurance are all gonna say.... YOU did this illegally. His begging forgiveness won't even enter into the equation on your behalf. Sure, he might feel bad, but his lawyers will tell in the blink of an eye.... don't do a thing or he'll be held responsible and then they'll all tear into him.
Sorry, but this is where the buddy system stops. Friends go drinking and out to dinner together, but they don't work without permits and certainly not under the table. Get caught one time and you'll completely understand why I wrote this.
You do what you want. I wrote this so no one else tries a dumb stunt like this that have lives and businesses on the line and can't afford an unaccountable mistake, problem or accident.