I do want to stick my nose into this to say a couple quick things.
1) I think it's really hard to determine who is same-sexual-and-emotional-level, or at least, level enough that people can be sexual together in a way where it's fully consensual and no one is being exploited. Our legal system mostly only considers that for young people when it comes to age-in-years, which isn't the best rubric for this, especially when it is the ONLY rubric (not considering say, intellectual capacity, or varying interpersonal positions of power and vulnerability: like, how/in what context did your friend even meet this person?). It is possible that a 12 year old and 16 year old CAN be sexual in ways that are consensual, but like Mo said, I'd agree that in the grand scheme of things, that can be a pretty wide age -- and often also life experience -- gap, so it'd be pretty unlikely. And for sure, in many areas, that likely was probably outside age of consent laws, besides.
2) What do YOU want to do about this? It's tough when a friend shares something like this, especially if it's being shared in a way where either it doesn't seem like they have any awareness they may be talking about an abuse they engaged in OR if they do seem aware and are talking like it doesn't matter or, as sometimes can happen, are even bragging about it.
Do you want to talk to your friend about any of this? Do you want to set a boundary with them of some kind? What do you need, and how can we help you figure out where to take this from here?
Never doubt that a small group of thoughtful, committed citizens can change the world. Indeed, it is the only thing that ever has. - Margaret Mead