Frankly, it would take a lot of courage to accept an execution so calmly. I don't think that Fredo had it in him and few otherwise brave people would have.
A more pertinent question is, IMHO, did Connie know? Her antennae seemed to be pretty finely tuned when Carlo was about to be whacked. That is why she tried to protect him via making Michael godfather to their kid. Which, of course, didn't work.
So, she had to know that the reconciliation that she had seemingly mediated between Mike and Fredo didn't mean much either. And when Tony suddenly got called away and Fredo was left with Neri... yea. She should have been aware what was happening and gone along with it, I guess.
When it comes to Connie I think maybe not right away she suspected Fredo's murder but as she got more and more involved in the criminal side of the family as we see in part III she probably put the pieces together. When it comes to Fredo people can say he was dumb which he was but i mean come one the guy knew what goes on in the mob so he knew that his days were numbered whether it nwas that day in the boat or a week from then
