Prometheus stays respectfully silent while Loki gives his instructions to the teenaged girl, then lets himself be pulled aside so that he can get his answer. It is terrible news to hear, but not entirely unexpected. Thanos had been a looming threat in Thor's universe for a long while. What a tragedy that he struck so soon, while Thor's people were most vulnerable.

He makes note of the way Loki speaks. Hoo boy, that's some trauma there. He keeps it in mind while he replies, his voice low and reassuring. "Then we will make sure that they are well-cared for. Loki, on my honor, I will do whatever I can to aid you and your people. You are not alone in this."

Although he is willing to bet all his drachma that Thor is still alive and finding a way to rescue the others. His friend is far too noble (and stubborn) to give up on his people.