When I'm describing cross contamination or cheating on the Gluten Free diet, I like to make a distinction between reactivity and sensitivity.
All celiacs are highly sensitive to gluten, meaning they will have microscopic damage from even small (think crumb size or less) amounts of gluten. There is a study floating around out there that states that 90% of celiacs will have intestinal damage from 20 parts per million of gluten contamination in their food. The other 10% will have damage from smaller amounts of gluten. As celiacs, our sensitivity levels are similar. Essentially zero tolerance to avoid intenstinal damage.
Then there is how a person subjectively reacts to the gluten. How does it make them feel? Some people get a little gassy from a random crumb. Others, feel pain for days or weeks from foods processed on the same lines as gluten foods. There are some celiacs who stop reacting to gluten altogether after a period of time (however they still have intestinal damage from eating gluten). Everyone's reaction is different. People who are regular cheaters on the diet will often not notice how they are reacting because it just feels normal for them. For this reason, I often advise people to be extremely strict in the beginning of the diet, so they find out how good they can feel totally gluten free and then when they add back slightly riskier foods like gluten-free oatmeal, grains, foods from shared lines, restaraunts, and friend/family cooked meals, they will be better able to tell if they are reacting.
Now, why would you and your wife care about occassional intestinal damage? One study suggested that undiagnosed celiacs have a 2x chance of dying from all causes, but that celiacs who are intermittent consumers of gluten have a 6x chance of dying from all causes. The same blood tests used to diagnose your wife can be used to monitor her compliance with the diet. 6 month then annual checks are sometimes recommended. The numbers should drop over time and eventually fall into the normal range if she is able to get all the gluten out of her diet. Cross contamination happens to all of us, especially in the beginning. It's a lifestyle change once you are diagnosed, but with time and education, most of us are very happy with our gluten free foods and healthier bodies.