I don't think that Elrond took Isildur into the fire mountain in the book. The last combat between Elendil and Gil-Galad, assisted by Isildur, Elrond, and Cirdan on one side, and Sauron on the other, was on the slopes of Mount Doom, presumably in full view of the armies of Elves and Men.
Only Elrond and Cirdan were close enough to see that Isildur picked up The Ring, and they didn't shout loud enough for the armies to hear what they discussed. But if Elrond and Cirdan had fought Isildur the soldiers in both armies would have noticed it and a civil war might have erupted in the moment of victory.
I suspect that instead of asking Isildur to follow him into the tunnel in the volcano and then asking him to toss The Ring into the lava, Elrond told Isildur out on the slops of Mount Doom that destroying The Ring would be a good idea and Isildur said he would keep The Ring. After Isildur's refusal Elrond would have no reason to go inside the dangerous and sinister tunnel and Isildur would not have accepted an invitation to go in with Elrond - The Ring would make Isildur suspect Elrond intended foul play.
I don't know if there is any canonical proof in the books that Elrond and Isildur never went inside the mountain, but there is certainly no evidence that I remember that they ever entered the mountain.
Elrond and Cirdan didn't know what evils would come from not destroying The Ring, and they were comrades in arms with Elendil and his son Isildur for seven years in the siege of Barad-dur. They had just gone through a terrible and almost unwinnable combat and suffered the trauma of seeing their king Gil-Galad and their great ally Elendil killed, and were experiencing the euphoria of unexpected victory and also post-combat exhaustion and were not in the mood to quarrel with their comrade Isildur - who had just seen his father killed - about their vague misgivings of trouble.
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