OP Prior to last night did this 'friend' of step-brother know where your friend lived?
As very clearly he does now and he will also know that you will not be there forever. So in blunt terms:
1: He is a predatory offender;
2: If NOT reported now & when so so much prima facie evidence against him, the boost to his confidence in how 'right' he is in believing he will not be caught WILL spur him on.
3: He does (now) definitely know where your friend lives and lives alone;
4: Right NOW, he knows he has not been reported even with 2 of you there;
5: Your friend therefore seems to me to now be at THE most immense personal risk.
There is so so much about this that is chilling, not least the fact he offered to walk you home in safety and then transformed into the perp. That is pre-meditated and psychopathic behaviour - that coupled with 1-5 above means you really have no choice but to report him.
Your darling friend's judgement and decision making will be hugely askew right now via last night per se and the (truly horrendous) triggering, but ours - all of us here - is not. You have multiple posts telling you the same thing and you must now act on that given the (unspeakable) possibilities that 1-5 HAVE now created.
When you are 'in' a sit'n it can be hard to see outside of it and you will be both in shock/trauma at last night as well as beset by immediate concerns for your friend. We are NOT 'in it' and you have many people speaking as one - please please listen to us; if you don't your friend may well be the person who remains at the greatest of risk, let alone others he WILL attack.
You have already said it yourself *'Future rapist for sure'. You KNOW what you must do so please please do it?
And know that victim support will be immediately available for your friend ESP. given what she has already endured, but for the love of God please don't let her be this creatures next victim which is sadly now a very real possibility 

