The entire healing model for the expansion is "mana matters" and it's also something that was too quickly becoming overcome by simple gear scaling. As it was, the entire concept of watching your mana would have been outdated by the end of T12.

More gear means more intellect and more spirit. More Intellect and more Spirit means more effective replens, more effective Innervates, more effective totems, more effective heals. In order to keep the "mana matters" paradigm, Blizzard needed to do something. They could ramp up raid damage (eventually going back to the ICC situation of constantly spamming everyone to keep them from being one-shotted) or nerf big regen cooldowns like Mana Tide/Divine Plea/Innervate, raise the base cost of spells and tone down base regen through talents, etc. Mana Tide was Public Enemy No. 1 in that and unsurprisingly the first thing to hit the chopping block. Innervate also got thrown in front of the bus and you can expect others to follow. There was a blue post that even outright said that Paladins have way too much regen right now. They're in line too.

IMO, this is a good thing for Shamans since they wont feel as much of a need to stack Spirit over throughput stats for the good of the raid. 200% of the average T11 Shaman's Spirit is still substantial, even if it's not ZOMGAWESOMEWATCHMYMANABARFILLUPINTWOSECONDSROLFLOLOMAO!!.

The bad news is that healers of every spec can expect to see a new string of nerfs with every patch. It's just one more stage in the Mana Wars that are likely to plague the entire expansion. As a DPS I've accepted the fact that I will always need to consider my Hit/Expertise ratings. Healers should likewise accept the fact that managing their mana is something they won't get to out-gear while Deathwing is alive.

Besides, rather than telling your Shaman who (by your account) really knows what he's doing and has been playing the same character for years to pick up, level, learn and gear an entirely new class because Mana Tide got nerfed, why not just let him roll Ele or Enhance for a while? Class Balance has always and will always be a thing of constant flux. Forcing someone to reroll as a FoTM healer because of a few nerfs THAT HAVENT EVEN BEEN FINALIZED YET and we can't see the end result of is downright ridiculous.

Our only Resto Shaman is a fantastic player and, though he may fall a little behind the paladins, priest and druids due to simple balance issues, it's a difference of 2-3% healing done on average, not 25-30%. If your shaman is having that much trouble keeping up, then it's likely he's gemming, enchanting, gearing, and reforging for spirit in every possible way to make his Mana Tide more effective (which, judging by your comment, he's probably come to feel is his only ticket to a raid spot), instead of looking at throughput stats to make himself a more effective healer.