Offhand, we'd say yes, she'll be able to handle the casinos, especially if you stick to the Strip megaresorts. Over the years, the casinos' smoking areas have gotten smaller as the ventilation systems have gotten larger. For example, reportedly, Luxor's air-purification machinery is the largest in the world. Some of the smaller casinos can still get pretty smoky, but the major ones have noticeably fresh air.
On the other hand, all it takes is a single smoker nearby to irritate someone who's senstivie to cigarettes, like your wife. So we suggest you actively seek out the non-smoking areas to play in and don't venture too far away.
As for restaurants, they're the same in Nevada as they are everywhere that smoking is still allowed indoors. There are smoking and non-smoking sections and if you ask to be seated farthest away from the smokers, you might have to wait an extra few minutes for a table, but you'll be able to mostly avoid second-hand smoke.
