I am completely confused.
Why hasn’t your friend reported him to the police, or to the parish pastor? Your reporting it is only here say evidence–you have no way to know if the accusation is true or not. Bearing false witness is a sin, although if the accusation is false, this is on your friend, not you. But I would not run-off to the police half-cocked, without talking to your friend first. The best thing would be to encourage your friend to report the incident to the proper authorities. Your report would not likely be something the police could act on. You might want to have a very private meeting with the parish pastor to let him know what you were told. In any event, your friend is going to have to come forward.
The accused’s taking communion in a state of mortal sin endangers him; it does not defile the Sacrament for everyone else. I don’t get your reasoning here.
The priest, while he can withhold absolution as a tool to get the offender to turn himself in, cannot, under sanction of Canon Law, reveal the content of a penitent’s confession to anyone else. He can be defrocked for doing so.