If the allegation is true, hopefully your friend speaks up.
If not, you might consider whether you have actionable information yourself, any professional obligations you have (e.g., if you work in science/health/safety yourself), any societal obligations, whether reporting the allegation would be betraying a trust, and what the calculus is there.
If not, you might consider whether you have actionable information yourself, any professional obligations you have (e.g., if you work in science/health/safety yourself), any societal obligations, whether reporting the allegation would be betraying a trust, and what the calculus is there.