I would venture a guess that it's the lingering smoke on mother's clothes, skin, and hair because that's exposure to secondhand smoke, even if it's not as blatant as the mother is smoking in the same area as the infant (which is obviously a very bad thing to do).
My husband smokes, and we co-sleep, BUT he always puts an overshirt on, only smokes outside, wears his hair back in a ponytail, and when he comes back inside, he takes off the overshirt, washes his hands and face, and brushes his teeth. He changes clothes completely before bed. And he never handled the baby without going through the "de-smoke" ritual, and he doesn't smoke in our car, or around the baby if the baby's outside.
That was our agreement, since he couldn't quit entirely. And we've been very careful about avoiding all other potential SIDS factors, but I'm not hugely troubled by the fact tht my husband smokes, since he's very conscientious about the effects of secondhand smoke around the baby. I'm not pro-smoking or anything, but I understand how hard it can be to quit for some people.