But she knew he was his second by the time she poisoned him. I totally understand Alex's anger. I also understand Abigail's decision, but she had to know there was a chance he broke with her and do not forgive her.
What I do not think is that Alex was angered because of his hurt pride.
This.
Part of why this works so well is because their are multiple layers to it. Everything I quoted is true and Abigail admits it to herself in an inner monologue. There is also an element of this, Valesco hurt Abigail and Bayard wanted personal satisfaction. Bayard is a proud duellist and Albions champ. To have the fight taken from him rankled his honor a bit. The fact that Abigail poisoned him, robbing him of his honor, just to send his best friend off to his death, was a killer.
But, this was not just about Abigail protecting Bayard. Abigail and Grimm had fairly explicit orders not to allow the duel to happen. Aurora counted on Bayards stiff honor to carry him to his death. They did not account for Abigail using treachery or Grimm being the one guy who could concoct a plan to win the duel with extreme levels of misdirection and guile.
It's good writing because it didn't rely on anyone being a superhuman. Just talented people playing to their strengths