You can't attend theoretically or any other way. HE'S MARRIED unless he's gotten an annulment.
This does not fall under civil duty. Social maybe.
Attendance would condone his sin thereby causing scandal.
Parents of divorced children have to make this decision all the time and it breaks their hearts.
This is just a friend.
The civil duty is to his wife; "keeping the peace" within a family, so to speak. If OP is literally faced with a choice between attending this service or getting divorced, such a situation is the archetypal situation considered by canonists and moralists as, if any, a situation that could allow passive attendance.
The OP asked if the material he found was a Novus Ordo innovation. It is not. It is
correct to say that canon law
does not forbid attendance of an invalid marriage. It forbids
active participation in non-Catholic worship. This includes weddings and funerals. And I'm not even talking about the Novus Ordo canon law, I'm talking about the 1917 CIC, which the OP, as a sedevacantist, is following.
Of course it's true that scandal, perversion, etc. must be avoided or rendered remote in either event. Hence the recommendation of sitting quietly in the back with a rosary. Also, the question if the wife would be satisfied attending the reception and not the ceremony itself.
It's very confusing, of course, especially when these things happen in the Novus Ordo.