If it will hurt the value of your property and put you or your family in unsafe condition, you turn him in........

Have you tried talking to him about your thoughts on his indecent proposal of not having a permit?

I agree, blow the whistle if need be. When an inspector comes to in when he has a buyer or you have a buyer down the road and there is something obvious the inspector spots it is made known to the potential buyer and that could pose some resale issues along with what you mentioned above!

Worse comes to worst, you could offer to purchase the permit yourself and buy yourself some peace of mind.

If you have to continue living in harmony with this person for anything like an extended time, calling in authorities will put a definite strain on the relationship so please consider that your last resort. But do tell him it is an option for you to call the city.

ETA, or, lay in wait for his real estate agent and give him/her a friendly heads up.

You said it “doing something illegal”…you need to ask?

Remind him that if the stairs are dangerous or not up to code, he might get offers, no legit home inspector is going to let something go to closing on his license.

Tell him to either do it right or don't touch. If these stairs touch your unit in any way, he needs written permission from you before any action is taken.

You can remind him that a city Stop Work notice will also serve as a Stop Sale notice

You sometimes don't need a permit to repair stairs, only to replace.

Is any of the current stairs staying?