I doubt the ex-gf's accusation will go any where, especially since it seems she was not serious about the relationship.
Pay heed though to:
That is from an ICE agent. She vehemently disagrees with the consensus view on Visajourney and other web sites that marriage fraud isn't prosecuted. She says she does investigate each case she gets, and here for the first time she notes she got one sent to prison.
So if your friend is in a true bonafide relationship with his fiancee, that's wonderful, it will be fine, and best wishes for him and her.
But if not, then he doesn't want to meet up with the wrong ICE officer.
They should not do K-1 though. CR-1 is better, and don't enter the U.S. until married 2 years to avoid I-751.