It's very easy for a girl to pick up bacterial vaginosis, more often than not that's what that smell is. It's just an imbalance of their pH inside the vagina. Often happens when guys with dirty fingers, or dirty fingernails put their fingers inside, could be from saliva from the guy's mouth, or even from a penis uncovered inside, including when a man comes inside the girl's vagina. I would still suggest based on multiple reports that she do a full STI screening, for her own health more than anything.