Unaddressed ethical violation I know about but have no connection to: Do I report it?

I took action... After carefully reading our honor code and the procedure for reporting misconduct I felt more compelled to act. Ultimately, I think falsifying data is one of the worst things you can do as a scientist. I just couldn't stop thinking about the new department he is in, the new advisors he has, and the new cohort he is apart of... all ignorant of this offense. If I was his new advisor, I would want to know about this. So out of respect for them and for my institution I knew I would act. The question was how?

It turns out my university has a protocol for reporting misconduct anonymously, which is great. But rather than go that route, I actually asked the PI if we could meet. PI was taken aback because, although PI knows who I am, we are not at all working together. In our meeting I simply explained that...

"I feel compelled to act in accordance with the honor code given that I am aware of the situation. But once I report the incident, you will likely be the first person they contact. So I wanted to come to you instead, out of respect, and explain how I felt regarding the situation and basically find out if you prefer to take care of it or if you prefer for me to move forward acting in my own good conscious."

PI was surprisingly kind about it, empathetic even. PI was very responsive to me explaining that I mostly felt bad for the new people this person was working with, that they were ignorant to his lack of ethical integrity--something the PI had not considered. We discussed how the PI wrote the RA a letter of rec to get into the new program, and I could see a realization on the part of the PI--specifically, that he had a responsibility to let these folks know that he no longer stands by his characterization of this student. In the end, PI assured me that we were in agreement, that taking official action through our university's research office was the appropriate next step. I apologized profusely for meddling but PI assured me I did the right thing and that I would've been equally right to have reported it as well.

So in answer to my own question, I would say that...

• First, read your honor code carefully to find out what responsibility you have to report incidents you know about. (This is why S.Diaxo's answer is incorrect; not only is there something I can do about it, I am technically required to do something about it).
• Figure out what your options are for reporting misconduct, it may be the case, as at my university, that you can simply report it anonymously and that they will look into it and you do not need to be involved.
• If you feel compelled to report something, but you want to be respectful to the people it might involve, consider talking to those people directly (in a respectful and non confrontational manner). That worked for me, and I'm glad I don't need to be the one to report it.

Thanks everyone for your discussion.