On a related note, since this thread was ALSO kind of just me complaining about one of my players...why stop now.
This same player actually threatened to stop playing D&D entirely earlier this week(something he only said because he was angry. He showed up for our Wednesday D&D Expeditions session). He was playing a Warlock and he made a comment about how everyone got disadvantage against him because he was standing in his Darkness spell. I pointed out to him that it wasn't true. Darkness makes you blind which means you give advantage to everyone who attacks you. They have disadvantage because they can't see you, but that cancels each other out.
He said "That's the stupidest thing I've ever heard. This game is dumb if that's the case. I'm standing in darkness and people get no disadvantage to hit me."
I said "Well, technically, they still have to guess which square you are in, so if they don't know then there's a nearly 0 percent chance they hit you. But even if they know what square you're in, it kind of makes sense that the two cancel each other out. They are firing blindly into the darkness but you have no idea the attack is coming so you can't even begin to get out of the way. So, their chance of hitting is about normal again."
He said "No....no, that's stupid. That's extremely dumb. I built my entire character around hiding in the Darkness spell so people would have trouble hitting me. Now I'm going to have to change my character entirely to a different type of warlock just so I can see in darkness so that I can cancel out that penalty. I might as well not play this game any more if this is the kind of stupid thing that happens."
He also tossed a bunch of dice around in anger. The tone doesn't come across well in text but he was REALLY pissed off.
Which just kind of goes to show that the issue I'm dealing with having this player around amount to the fact that the player wants everything to go his way 100% of the time. If I don't rule how he wants me to he gets angry and starts taking a fit.
I disagree with your player's attitude.
I agree with his theory on Darkness.
Although by RAW, your ruling is correct, this is one of the house rules in our game (and we only have 5 rules). RAW is somewhat incomplete here (for stealth, hiding, invisibility, vision, blindness, etc.).
As an example, NPC up in a tree 50 feet away shooting arrows at PCs. PC spell caster casts Wall of Fog around PCs to protect them. The spell does little since the PCs are not hidden. In order to move away from their original squares, the PCs would (theoretically) need to stealth. Theoretically, they are making noise otherwise (or not based on DM fiat, hence, incomplete rules). So, the archer shoots at where a given PC was (or where he hears a PC moving to, or not). He is shooting into an area he cannot see, but his chances of hitting are exactly the same unless the DM rules that the PCs can move away from their original squares without making noise and without needing a stealth roll.
So, different DMs can easily rule different here.
Now, let's take the same example, but one of the PCs is in a square behind the Fog Cloud. The archer has disadvantage in this case because the PC can now see. It doesn't matter that the PC really would not be able to see the attack coming (an arrow moving 5 feet from fog to non-fog takes about 0.03 seconds to get there, way faster than any human could ever react), it just matters that the PC can now see.
Oh and PCs in the Fog Cloud shooting at the archer? They have disadvantage, but can fire at his exact square because they saw it previously. The fact that they are totally blind and shouldn't know north from south at all does not prevent them from precisely pinpointing his position.
Now, many DMs like the flexibility in these rules. Personally, I think that they are incomplete and can easily cause problems like the one you experienced with your player. A Fog Cloud or Darkness spell SHOULD protect PCs from ranged attacks. They do not, hence, these particular rules are a bit stupid (or minimally, weak). IMO.
