His wife went to the United States on a visiting visa and has decided not to return.
As it relates to my friend, his mother is a naturalised United States citizen and is currently filing for him.
Once he is divorced, he should advise the National Visa Center (NVC) with a certified copy of the final divorce order and ask to be moved to the F1 preference category.
Visas are currently available in that category for people who were filed for before May 22, 2014.
You didn’t indicate when your friend’s mother actually filed for him, but if by chance his divorce is not final when his consular processing of his visa begins, he should proceed by himself – along with any children – and advise the NVC that his wife will not be included in the visa processing.