I was told that one way to help a young child break out of a tantrum is to ask them an odd question, something like “What color shoes are you wearing?” It does seem to work sometimes, usually by annoying them so much that they forget what they were upset about in the first place. I can well see it working for someone who needs to get out of a mental spiral.
Sometimes their emotions have gone so far past reasonable that the first thing you need to do is bring them back to the point you can actually reason with them. After that, yes it’s really vital to take their emotions seriously, they need to understand them and trust that the people around them will take them seriously, but they also haven’t yet built the skills to moderate their own feelings, so sometimes you need to add those externally.