She lying and he’s believing it

Hey ladies, so over the weekend my little sister (16) had a guy over who is not her boyfriend, but is one of her boyfriends good friends. I heard them in the basement making out and before I could get any kind of picture or video as evidence, her boyfriend called and they stopped kissing. Her boyfriend is such a great kid and does anything for her. My sister on the other hand is a bitch, defiant and a chronic liar. I told her boyfriend what was going on and he believed me... at first. Then he confronted her about it and she kept telling him that she would never do that to him. Is there any way that I could make either her or the guy tell the truth? It’s upsetting me that her boyfriend had no idea what’s hurting him... please help!