YD 103 http://hebrewbooks.org/pdfpager.aspx?req=9147&st=&pgnum=265
It says: If you see someone wearing shaatnez doraisa knowingly, you are supposed to jump on him and rip it off him, even in the marketplace. Even if it is your Rebbi.
But, if he does not know, then you can wait to tell him until he gets home, because of his dignity. (not because of tznius -popa)
But if it is only shaatnez drabonon, then you can wait until you get home.