Statistically what are the odds that a tumor on a non-smoker's lungs will be benign?

It depends. The risk of cancer depends on the characteristics of the lesion on the lung, but overall the majority of nodules in the lungs of nonsmokers (and even smokers) will turn out to be benign. Even in high risk current and former smokers being screened for lung cancer, 96% of nodules were benign.
Better than 50-50. 65% of solitary chest xray nodules were benign in smokers and non-smokers in a study published in 1997. http://www.ncbi.nlm.nih.gov/pubmed/9129544 the group did find that smokers had a higher likelihood to have a malignant nodule, but did not report the results in the abstract.