Note that allowing iframes directly in a post content area can be dangerous, that’s why they’re stripped out for security reasons.
Super admins on multisites or administrators on single sites have the unfiltered_html capability, and can insert anything they please into a post content area, but this is dangerous. For example, granting your author this capability would allow them to copy paste javascript into the post content area. I would be wary of bitcoin miners, if not credit card stealers, or router malware.
Instead, have you considered using an iframe shortcode? If the iframe is for a video or another kind of embed, then OEmbed would be the appropriate path forward. If it’s a Youtube video or common service, it’s likely that’s already supported, in which case simply copy pasting the URL of the content to be embedded on its own line should turn it into an iframe embed automagically.
iframe embeds implemented via OEmbed work with the shortcode automatically, and you may even find that the block editor creates blocks for you