He doesn't stop her, actually, because Geralt is long used to people chewing him out and it's never deterred him. He waits patiently for the words to fade.

They didn't.

She's not the only one capable of untraceable portals. They just come at an exceptionally high cost. Besides, if they catch wind that he's exited the borders, it'll fall on him alone, and he has no intention of telling them where he went with the stone. As far as the man he purchased it from is concerned, he's searching for something in the Badlands, or perhaps he has vague ideas of contingency plans for defecting, or he means to snuggle it to someone in another nation. It doesn't matter much what, only that Geralt's open interest in the Singularity or any magic as a whole has been nonexistent since he landed in this world.

It won't hurt me, either.
I've entered the crater before.

Some large cats have looked at your body like dinner, however.

She may feel safe inside the Singularity, but he's not certain what it's supposed to do if a wyvern swoops down to make her its next meal. Ciri's spawned strange plant-like monsters here, too; he doubts they're completely eradicated. Unlike people, beasts seem to enter the crater just fine.

In any case, he's here. She can be upset at him about it all she wants when they're home. He's more concerned about her not returning home at all.