The law does allow an individual to apply for lawful permanent residency within the US if they entered on a visitor visa and later married a US citizen. There are many factors that determine whether or not the individual is eligible to become a resident (including financial, medical, criminal, etc.). In addition, if the government believes that the person intended to stay in the US at the time they entered on the visitor visa they could be denied or accused of committing fraud which would mean they would have to file for a waiver. If the individual is eligible, the paperwork can be filed as soon as the marriage takes place and they can receive work authorization approximately 90 days after filing. I strongly recommend that you consult with an immigration attorney to begin this process.