Frigga watched the whole thing play out just like in the nightmares, or visions, or whatever they are. She is frozen, she knows she should move, but she can't. Her sons spar for a moment over her sleeping husband's glowing bedchamber. It is only after her youngest blasts her eldest through the side of the palace that she can move.
Loki strides past her without making eye contact.
She can move again; she runs after him into the hall, "Loki Odinson, you get back here this instant!"
He freezes, sighs, and turns toward her, "Mother, I-"
She slaps his face hard enough that his cheek reddens, "Don't you dare lie to me, young man. What in the name of the Nine are you doing?"
He looks at her with sad eyes, "If you've ever loved me, don't stop me." He takes her hand, squeezes it hard, and turns quickly so she won't see that she has broken through his carefully constructed mask of anger.
She stays, rooted in place in the hall, her heart breaking as she watches him walk away. Only after he is out of sight does she return to her sleeping husband, take his hand, and cry. It is the only thing she can do. When he wakes, she tells him all she knows, but she cannot follow him to the Bifrost. He would never believe that her again when she said she loved him. She lets Odin go alone.
When he comes back with only one son, she hopes Loki never asked himself why she did not come.