What are the chances a district attorney will file charges in a sexual assault case?

My friend was with a girl who he met on the Internet, the same day they hung out. In text messages they discussed to do things like touching kissing etc. the only thing they agreed on was to not have sex. He picked her up and they hung out at his place she gave him oral sex for a couple mins as he played with her vagina. This was the furthest they went this all started because the girl wore no underwear initially and she told him this. Well after about 15-20 mins of this she wanted to stop my friend stopped and did no more the girl said I want to go home. So she got her things together and started to walk out , he asked her ok let me at least give you a ride home. She said no ill just walk, he said ok and left the situation alone. Hours later the cops showed up to his house and detained him then took him down the police station, for an investigative session with a detective. He gave his story and then the detective let him go. Nothing has happened since then, this issue took place 1 month ago in Salt Lake City, Utah. He talks to the detective weekly to figure what charges were they were trying to file against him recently a couple days ago he just learn that the victim had not been answering the phone or accepting calls from the detective. The DA told the detective a couple weeks ago to get more interviews from victims and witnesses but he said none would answer. My friend is worried sick waiting to hear back what the verdict will be. The Detective recently sent back the files letting the DA know he couldn't get in touch with victims or witnesses. My friend is waiting to hear what the DA will do. So we are curious will they just dismiss the case due to a lack of evidence or no victim ?