My (27M) grandmother (84F) shared something painful about our family history with me. Should I ignore it or try to discuss it further with her?

I must start with some backstory. My (27M) grandmother (84F) is originally from Chile. My grandfather, who I never met, worked as a lawyer and represented a left-wing party when the military dictatorship was established in 1973; shortly after that he was taken in for questioning and never seen again.
It was determined only decades later that he died under torture at the hands of military agents, though his remains were never located. My father was barely 2, my aunt was like 3 or 4, so they barely remember their own father, and I didn?t hear much about him growing up.
It was like my grandma chose to close that chapter of her life. She found a new partner and left Chile with both her kids two years later ? this partner, who?s now deceased, is the man I grew up calling grandpa (side note: he and my grandma were never legally married, because my bio grandfather?s death certificate hadn?t been expedited, so she was still technically married in Chile, her husband was considered ?missing?).
Well, yesterday I stopped by my grandmother?s house to see how she was doing (she?s independent and still lives by herself), and as we?re talking about some random stuff I showed her a recipe of a Chilean dish that I was thinking about cooking. And then she said something that caught me off guard. She said: ?that?s what I served to the men who killed your grandfather?.
And I didn?t know how to respond? She had never mentioned this to me before. She then proceeded to tell me that, when the officers came in to take my grandfather for questioning (with the excuse he?d be back by the end of the day), part of the group stayed behind with her and the kids. I?m assuming it was to conduct searches if his interrogation led to something, or to prevent her from getting rid of incriminating stuff, because they didn?t know if she was an ?accomplice?.
She said she was cooking lunch when officers arrived. Then, in minutes, he was taken, and some of these strange men stayed with her. And she, trying to act polite and to disguise her desperation, asked them if they would stay for lunch. And they said yes, and ended up eating her food, like they were guests. The food she was preparing for her husband. That killed me!
I never knew that. I mentioned that my dad never told me about this. She said my dad doesn?t know ? he was there at the house, but was too young to remember. She also said the officers stayed put for four days, sleeping on the couch, always denying to tell her when my grandfather would be back (it was supposed to be an interview over a couple of hours). She spent all this time cooking for them, practically held prisoner in her own home with two small children. And when they finally left, she knew immediately that my grandfather had been killed, as in: the officers were dismissed because the superiors told them my grandfather couldn't give them anything of value.
I?m very touched by all of this. I never really thought about what she went through. My first reaction was shock, and I didn't quite know how to respond, but she changed the subject shortly after and I didn't know how to approach it again. I wonder why she chose to say something to me now, after all this time ? and something that even my father doesn?t know, and I don?t know if I should tell him or not (she didn?t ask me to keep secret, I just feel it would be some sort of betrayal of her trust to tell him). Most of all, I don?t know how to react when I visit her next ? should I approach the subject again, or pretend she never told me anything unless she brings it up again. I appreciate any advice.
TL;DR: My grandmother opened up to me about the day my grandfather was taken by dictatorship agents never to return, and I don?t know how to react to this revelation.


Source.