Advice for young girls on dating
If they were good, she was happy, going to class, doing good.
If he was ignoring her, she was depressed, missing class, consumed with anxiety.
Usually the older man has more power simply from the fact that he is older, and they usually have more money and resources than the teenager or her friends.
Older men usually have had more sexual experience that often include risky sexual behaviors.
The young teenage girl is not likely to question his sexual behavior or health status, and even if she did, she is likely to take his word for it instead of asking him to go get tested with her.
A lucky few end up in marriage, which is often proceeded by a child and continues with multiple children, poverty, physical and substance abuse. Let him know that if he really cares about her, he will abide by your wishes since you only have her best interest in mind and he should too if he cares about her.
Depending on the age of your daughter and the age of the man, it may be appropriate to get law enforcement involved.