US VISITORS VISA

My sister is married to a Mexican citizen in Mexico, but they want to come visit us in the United States this summer. I told her that if a spouse tries to get a visitors visa they are almost always denied because of the likelihood of overstaying. Realistically, would the consulate know that they are married? If their marriage is filed in Mexico and not in the United States, is there a possibility he could just be visiting old friends (because he also used to work in the United States and has many friends in our hometown)?

If anyone has any advice, greatly appreciated. My family misses him very much.