Interesting one this - Roy Castle's case is often held up because despite being a non-smoker he died of a lung cancer strongly associated with smokers.
The claim therefore was that he must has contracted it from second hand smoke.
However this is certainly not proof, it's an assumption based on available evidence.
I've seen figures bandied about that 99% of these cases contracted it from smoking - does that mean that 1% got it another way or via passive smoking? - I don't think anybody really knows.
It's a case of statistics and increased risk getting from that to a causal link is pretty tough - especially when the tobacco industry are trying their best to muddy the waters see here:
http://www.bupa.co.uk/health_information/html/ health_news/270503smoke.html