Well, basically, as long as the easier difficulties had fewer (i.e. a smaller or different drop set) rewards, I don't think leveldowned versions of things would be detrimental to the game. If the different drop set included lower-level pieces as well, this would actually be a potentially viable way to expand the level and scope of content in the game, I think. (I know I'd like to do a lower leveled Ifrit fight as soon as I was high enough to be useful in the party, especially if I got drops which would then benefit me as I climbed to 50 where I could do the Hyper Mode Ifrit fight.)

However, the idea that not all challenges in an MMO are for everyone is true - this was part of the core idea of the game, when "you can experience the main story content on a gatherer or crafter job" was still part of the spec - the Hyper Ifrit fight is for people who are trying to challenge themselves with a difficult fight, and get some pretty excellent rewards as a result. While the total content in this game is still /fairly/ low, your friends should be able to know their own limits and pick their challenges accordingly; if they can't feel like they're fully enjoying the game without doing content that they're not capable of handling for medical or personal reasons, this really isn't a game they're going to enjoy in the long term.