How to read chat comments on someone else's live stream video?

You can’t get that via IRC, but “chat replay” is available for VODs in the v5 API. I don’t think it’s available in Helix yet though. Here’s an example of downloading all chat and saving it into a JSON file:

Be aware that the chat replay might include messages that were added to the VOD itself, and will not include any messages that were purged as a result of moderator action.