The date printed on your GC is the date it was officially approved and issued - and from that date, you can apply for naturalization after three years. Or, I believe you can submit the application 90 days before the three year period is full.
The 2-year GC is the exact same thing as the 10-yr GC - besides for the 2-year vs. 10-year validity. The first one is conditional for people who have been married for less than 2 years, to make sure the marriage is bona fide and valid.
So - three years from the date printed on your FIRST Green card. You get the first one for 2 years, after those 2 years apply for removal of conditions, get the 10-yr GC, and after the first year has passed under the 10 yr GC you can apply for citizenship.
There is no international clearinghouse for marital records. Whether the previous marriage could come out in a background check depends very much on what sort of information was collected by each agency that collects information about people, and which of those agencies the US government has access to.
Your friends are naive and abundantly stupid for going through with this. There are documents, government records, and people that can confirm that the previous marriage exists. If USCIS ever receives any evidence from ANY of these sources then your friend will be hit with a material misrepresentation finding, her status will be revoked, and she will be removed from the US with a lifetime ban from ever returning. The threat of this happening will follow her for the rest of her life, even if she eventually becomes a US citizen.
Further, her new husband now has some serious blackmail evidence against her. If she ever crosses him and they end up divorcing then he can pull the plug on her life in the US with a single letter to USCIS.
