I know that quickened spell is one of the Sorcerer's best metamagic options, but i did have a question about the legality of an action one of my players took. He cast call lightning the turn before (using a revised storm sorcerer we found), and did the call lightning shenanigans. The next turn, he used his action to fire off the bolt, which I ruled as "not casting a spell". He then used the quickened spell option to cast a lightning bolt. Is this like a common sorcerer thing? I also know that Zac Oyama did almost the exact same thing in NADDPOD, just using different spells. 

If I’m reading it correctly he did the following 

-Quicken cast Call Lightning

-Use action on same turn to cause another lightning strike

-Next turn use action to cause another lightning strike

-Quicken cast Lightning Bolt

if so then yes, this is RAW correct. 

Now typically speaking Sorcerers don’t have to many spells that work like Call Lightning. They get Witch Bolt…which sucks. They get Maximilians’s Earthen Grasp which can be used to give the restrained condition. At higher levels they start getting a few like Telekineses and SunBeam so yes, that is a common Sorcerer thing, but typically not at levels 5 and 6

Thanks a bunch. Again, this was a revised storm sorcerer that included bonus spells. One of these spells (at 5th level), was Call Lightning. That's how he was able to do it. WHile it did eat through his sorcery points, he also basically nuked the boss.

This is a perfectly valid use of Quickened Spell, and it's possible without homebrew if the player multiclassed as a Tempest Cleric or a Druid, though of course that would limit the number of sorcery points they had to throw at it (and require a higher level character to get to that stage).

There are some similar combos that are possible without multiclassing, for example if you pick up inflict wounds (easy to do now thanks to Shadow Touched) then you can cast shadow blade at 3rd-level and attack with it every turn while quickening an upcast inflict wounds to deal some hefty melee damage. Obviously the lightning combo will do more overall if you've got the targets for it (and no allies in the way) but for single targets the damage is broadly the same, except that the melee option can get critical hits (and shadow blade can have advantage), whereas the areas can hit more targets and can't miss (always does at least half damage).

An enemy with any form of lightning resistance though will quickly nerf it, and if an enemy's indoors (nowhere for the storm to appear) then call lightning may be useless. Using the combo is also reliant on the enemy being somewhere it can be safely hit without harming any other targets. There's also the fact that it's concentration (same weakness as the shadow blade option) so an enemy attacking the sorcerer (biggest threat by the sounds of it, so reasonable for them to do) can interrupt it. It's potentially powerful, but not overwhelmingly so IMO.

Yea I’m in favor or Sorcerers getting more spells and Storm sorcerers getting this spell specially, but it does provide a pretty good nuke phase a bit earlier than they normally would 

The other thing they were able to do with their remaining 3rd level spell slot was grab our downed fighter and thunder step out as a quickened action, then use the call lightning. It was pretty cool.