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.