That doesn't necessarily mean fraud. Was it annulled because the court affirmed it to be fraudulent or it was something else? Seems like some kind of love triangle.
Since you know this person, in your heart of hearts do you think your friend entered into a sham marriage with any of these guys for immigration benefits?
In a recent case (Bouarfa v. Mayorkas) the supreme court unanimously affirmed that USCIS can revoke an approved I-130 petition at USCIS's discretion without the need for judiciary review if it suspect a sham marriage. Even though Bouarfa was in bona-fide marriage with a new USC spouse, an ex-spouse wrote to USCIS stating that he entered into their marriage just for immigration benefits (sham marriage), presumably with evidence. USCIS revoked his approved I-130 to current wife and that's where his legal trouble began. Now Bouarfa could technically file a new I-130 but it will be battle against USICS all over again.