I do not know if she was single when she applied, if she was, and she is now married, then nothing to worry about, produce evidence of marriage and all is good.
But...the writer wrote
"My best friend wife won visa lottery as single , and they are both legally married since 2010"
So when did she win the lottery? If it was before 2010 she would have had her interview already, so I assume she won the lottery recently and entered as a single woman when it fact she was married.
That is why I have stated that if you put single on your visa and you were actually married, that is a lie to the US immigration, pure and simple. They are risking getting a visa so many people apply for honestly. Even if the visa is given, who is to say they can get a visa for their spouse later on?
Now if the women applied, and then got married since the application, then she just needs to notify them and follow the procedure that I am sure is in place for this.
I suspect however that 'single' was written when in fact they were really 'married'.
It is fraudulent to lie to them.
Q