Even if stats are provided in incidentally; a lack of prosecution or failed prosecution is not definitive proof that a rape or sexual assault did not occur.
Stats are up to 10% of reported rapes have not occurred, and most often, it is in circumstances like this one.
I started my career in criminal law, and I have witnessed false accusations.
Inability to successfully prosecute or a decision that it's unfeasible to prosecute at all - are incredibly common.
If it's a he said/she said, as in this case, yes. You can't automatically default to the girl was raped. She may have consented and regretted her actions after the fact (I've seen a victim's friends provide testify that stated as much). He may have got her drunk and she couldn't consent. We just can't know the truth.
A 35 ye old man who had sex with an 18 yr old girl (plenty of 18 yr olds are still at school FFS, going to school in a uniform) had extremely doubtful integrity to begin with ...
I never stated otherwise. All I'm saying is that whether or not the sex was consensual, given it can't be proven either way, is irrelevant to the OP's decision. Her decision should be based on his actions and his character.