Okay, my bad. I didn't know he was not participating in the embargo.

This is exactly how it works. People who are part of the embargo are responsible for making sure that anyone they share information with -- which should usually be nobody! -- are aware of and abide by the embargo.

you cannot be subject to an embargo of which you had no knowledge and took no participation in.

the embargo was already broken when pete received the information.

So if I read the post here and told someone, would I be breaking the embargo?

Yes, it is.

This is not a situation where "Ignorantia juris non excusat" applies.