I'm going to expose a (former) friend for sexually harassing me
Have you mentioned anything to the authorities yet?  Or talked to your friend about what they were doing to you?
I've been struggling with this situation for a while.
It can be bad to keep instances of sexual harassment secret, as this could encourage the perpetrator to harass others in the future.  In situations like this, if you can't talk to the person responsible, it might be a good idea to alert the authorities.
I want him to pay.
That's a totally natural feeling to have. But be sure to let the authorities handle this.