Don’t do anything tonight. Sleep on it first.
Good advice - tomorrow when you are fully sober, alert, rested (as much as you can) and can think more clearly, you can work out what to do.
I am not surprised that you are upset and angry - it took tremendous courage to go to the police with the details of this vile man's assault on you, and it must seem that it was for nothing. Unfortunately it comes down to burden of proof. If the CPS feels that there is not enough evidence to get a conviction (and even today, juries tend to think that girls were "asking for it", or that it was a crush he didn't reciprocate - even in the face of often overwhelming evidence) they won't take it forward.
This doesn't mean they didn't believe you - just that there is not enough evidence to convince a jury. (Read "The Secret Barrister" to find out how prejudiced juries can be, and how they often ignore even the judge's advice.)
You have spoken out - be proud of yourself. If anything further comes up against him (and it well might), then the authorities have been alerted - and you may be approached to give your evidence again.
Re: this 17 year old. Do her parents know that she is dating a man so much older? I wouldn't like this if it was my young still-at-school daughter, and I'd want to know.
I'm going to record the conversation and try and get evidence
Be very careful with this - it is a grey area re: legality and I don't think any evidence obtained like that would be admissible anyway, though it may give you satisfaction. TBH, I think that unless he was so arrogant he was crowing about it, he would be very careful and just deny, deny, deny anyway.
Please don't do anything that could get you on the wrong side of the law. He will slip up one day - let him hang himself.