What did she tell him? I'd think Scion and entities were apart of the hazard info. Also how they were awakened would increase internal conflict within the PRT. So what info did she give him?
I dunno how much of scion and Entities would be hazard info. Bar Armsmasters shard making him forget things too close to his trigger vision.
Cauldron was telling people, and everyone knew at the end. Considering she told him about cauldron (though in a little round about way) this is unlikely to be an issue.
Its also not particularly needed to explain endbringer origins.
Eidolon's shard just has the bits to also control the endbringers. But he doesn't have the right mind etc. to order them properly. Endbringers act based on Eidolon's desire for a fight to push himself.
Which considering how devastated he is at the end... He probably knows that much.
As for how good it is tell him about it...
1. She lies and says its a info-hazard. Even though she doesn't think it is. Given I don't think shes changed her micro-expressions, and thinkers, decent chance she gets called out for lying. Turning people against her.
2. She tells them that while it strictly would be allowed under her what she said, she wont tell them anyway because it would be bad. Probably gets her less trust in the PRT and could be used by those against her to go against her. She sort of pushed them towards going for another question, but stuck to her promise.
3. She tells Armsmaster the truth. Sticking more to her personality and morals. Armsmaster makes the choice. Either way Armsmaster chooses, Cauldron considers her a little more untrustworthy, and want a little bit more to get rid of her. Even if they wanted the sort of information she was providing. Because it doesn't follow their 'greater good' mindset. Also somewhat applies to other people in government.
Also possible Armsmaster suspected of mastering etc.
3.a. If Arms tells them who knows what happens. They hide it from Eidolon? Eidolon does something maybe good maybe bad? Makes things a little more chaotic. But that was his decision now.
3.b. Arms doesn't tell them. (Cauldron could force the issue if they wanted?) Well mainly no change in PRT, bar that people know the info is horrifying and likely not particularly useful. Though some people will be opinionated and believe they should know anyway or believe that she shouldn't have told arms in the first place if she expected something like this.
No good options really.
