Since when is ARR-SB story irrelevant?
I personally found it very relevant. As it was fun to go through and get the story.
Since, part of what FFXIV does well is that unlike other MMO's, it makes the JOURNEY important not just the destination.
Did your friend not pay for ARR? For HW? For SB?
Also, while the end of ARR story certainly feels like it takes forever to get through (God damn it Minfilia!) it will not take two years to actually get through to current content (Which is how long Shadowbringers will last for).
Heck, it's entirely possible that your friend can get through all of the MSQ from level 1 to the end of Stormblood before Shadowbringers actually arrives in 3 months time... Especially if he has a friend to help him get through MSQ required duties without obscene DPS queues...
If Stormblood is anything to go by...
Trust me, it's favourable if new players don't get to experience Eureka grinds because they're too busy playing through the pretty great MSQ content instead.
As that's the only real thing that you could "Miss out" on if you took your time to level. Maybe some Raids would be harder to complete, but if you have a decent FC and some friends it shouldn't be impossible to run through them during end of expansion lulls when everyone is overgeared to make them easier to do.
With Shadowbringers, they're introducing the New Game+ system, which will allow people to play through the MSQ again from the very beginning. Thus alleviating this concern.
Perhaps they should reconsider why they're playing the game then.
If their only concern is reaching end-game and spamming Raids... Well... There's plenty of MMO's that cater to that type of gameplay. If they only want to farm Eureka... Well... They need to go see a psychologist because they're not right in the head xD
Maybe it's just me, but I play this game to have fun. I found the MSQ fun to do. I never felt that my time was being wasted, even though I took a break from mid-HW to mid-SB and slowly did the MSQ alongside other side activities. I wasn't doing cutting edge raids or progressing at the same rate as everyone else, but that didn't matter because I was having fun along the way.
WoW also has pretty trash actual story. With its entire focus being on "Get max level, spam Dungeons/Raids until the next expansion, rinse and repeat"
Personally, I feel that the opposite would be true.
I feel it would worsen the experience if players were easily able to get spoilers in the form of jumping to the latest content and seeing all the story it tells, before ever experiencing the lead up to this point from prior MSQ.
That's why I feel that people who actually care about the games story shouldn't use Story Skip potions, even if they are implementing NG+ to allow them to replay the story.
Since it would cheapen the entire story from previous expansions when you already know all the stuff that happened because of what's mentioned in the latest content.