Who is the best tradesperson to seal open gaps between two homes?

Who is the best tradesperson to seal open gaps (small holes or crevices or unsealed floor joists) between two homes to stop and prevent air/*cigarette smoke moving from one basement to another basement in semi-detached house? Carpenter?

In my home, we use the basement as living and sleeping space. The neighbouring home has recently had a basement tenant move in - this tenant is a heavy smoker. For the first time in ten years, we can occasionally smell some cigarette smoke in our basement. The tenant often smokes outside, but with Winter approaching, indoor smoking is likely to increase.

It is not clear where/how*exactly* the smoke is coming from into our basement (other than knowing it MUST be from next door as we are non-smokers). At home while resting, working, socializing or sleeping, we don't want to smell cigarette smoke, have the odour on our clothing, furniture, hair etc., nor most importantly assume the health risks of breathing secondhand smoke.

I understand that it can be tricky to completely seal off or block up a shared wall. However, it is possible. I realize there's no legal REQUIREMENT to share air with one's semi-detached neighbours, and since it IS possible, with fastidious & attentive-to-detail effort, to cover every square centimetre of wall between two houses with either concrete cement, caulking, foam, steel or other non-porous material, who can I seek out to do this renovation/repair? It is my understanding that smoke will not get beyond a concrete fire barrier.

I am willing and must ensure that I do everything to protect my family's health. One of my children has a chronic health issue that it is unconscionable to add to with this discretionary exposure to secondhand smoke.

I welcome any feedback, ideas and recommendations.

Thanks!