It doesn't take much for an indictment. I think about all he had to do was say she claimed to have been raped. And that she had "injuries" consistent with rape. Bada bing bada bang.
Never mind those so called injuries are also consistent with consensual (which we know took place) it makes no difference. Hence the infamous ham sandwich.
We'll never know for sure what Liefong said to the grand jury to convince them to indict. The proceedings are entirely secret and there is no transcript. But we can look to the March 16 search warrant affidavit by detective Himan and Liefong's early (false) comments to the media about Mangum's medical report for clues as to the claims he probably made to the grand jury.
Indicting grand juries constructed as they are in NC hear nothing from the defense and, although they have the right (and responsibility) to ask questions and seek more evidence before indicting, they routinely rubber-stamp whatever the DA wants. They basically indict on faith that the prosecutor is telling the truth and has his ducks in a row. They do not test what he says, there is no record of the proceedings, and NC criminal procedure has no subsequent pre-trial hearing process for probable cause review after indictments are handed down and the arrest is made.
So there you have it. Indicting grand juries no longer serve the purpose for which they were formed hundreds of years ago.