Smoke sucked into the computer by intake fans can cause several problems. The ammount of time it takes to manifest could be weeks or years.
Smoke can dirty the lens on CD drives.
There are other possible negative effects of smoke enter your computer, but these were the first to pop into my head. I no longer smoke in the house. Here's another, the smoke can acctualy stain the computer case, keyboard, or any other components it comes in contact with or finnaly comes to rest on. I hope she quits smoking anywhere near the computer before something fails. Never had a problem.