I'm really glad you reported what you could, and I think it is a terrible abuse of power for the other supervisor to be "dating" the 18 year old. I have a couple of situations that came to my mind when reading this thread.
#1- A situation I know about where a man in his 60s preyed upon 18 year old girls at the rate of 2 per year for over 30 years. He was in a position of power over them, he'd groom them, have a sexual relationship with them, then move on to the next one. He eventually killed himself and his company covered up what he had been doing all those years. Many of those girls were profoundly disturbed by the relationship they had with him, which he would always say was their fault. They never knew about the others. I am one of only a handful of people who know this ever happened. And no, I wasn't one of the girls. Thankfully.
#2-Once I (stupidly) hired a bright young man who had a habit of blurting out personal information. Once he was alone with me and he blurted out that he had sexual relationships with every female boss he ever had. He was very attractive and likeable. I replied, well thank goodness that won't happen this time. That young man was confused by every single thing I did. He was scarred by whatever "management" he had before. He could not comprehend his handsomeness being ignored and being rewarded solely on his work. His work was very good too! He was just too far gone to understand that this was how the "real world" worked! He hit on my other employee, who liked him too, so she politely ignored him. He had a sexual relationship with a girl in another department. It was too bad, because he was smart and I really did like this boy. He eventually left the company, I believe because he thought we didn't appreciate him. So yes, these relationships DAMAGE people!
THANK YOU for caring about this young woman! She may not appreciate it now, but you helped her, and that was wonderful!