You don't get citizenship through being married to a US Citizen. The only thing that does is decrease the wait time from five years to three years. If she got her permanent residence based on marriage to a US citizen, then she likely got a two year conditional residence. If she made it through this period before the divorce, or she can show that the marriage was not a sham, then her permanent residence shouldn't be at risk. Even without naturalization, she main remain in the country as a permanent resident. Once she waits out her three or five year wait time as appropriate, she can apply for naturalization. Again, provided the marriage wasn't shown to be a sham to get immigration status, the fact she subsequently divorced shouldn't be an issue.