1) Yes. It's not usually linked to being ground-bound. He'll know they're within 30'. For precise details, he needs blindsight.
2) A ruined building would likely block line of effect, blocking blindsense, especially if there's a roof to block LoE from above.
3) It says range of 30'. I would include all 3 dimensions if he's flying. I would take a literal rules translation to mean beneath and above him, even on the ground. It's active at all times, so if someone comes close while he's asleep, it can inform him. He probably doesn't need to make a Perception check, either.
4) Blindsense isn't precise enough for targeting necessarily. It'd let him know what square to target that an invisible creature is hiding in, but said invisible creature still has concealment, so targeting can get difficult. Fireballs don't need targets, that's part of the fun, he just chooses a target square. Consult the concealment rules and line of effect.
5) If your player is being abusive, let him try to sleep at night. Moths, mosquitos, bats, owls, cicadas, crickets, PCs, earthworms: the list of creatures within 30' is tremendous. Then find the thread about sleep deprivation;)
Blindsense lets you know that something is present. Blindsight is the ability to pinpoint things precisely. Blindsense lets you know something is out there.
In reference to the Dragon Disciple's blindsense:
Can he fly 30 feet in the air and do a search pattern to automatically detect anyone invisible?
Can he detect these invisible people if they're in a ruined building? If they have a partial roof over their head?
Is blindsense a 30 foot radius around him? Is it active when he's asleep, so that he'll wake if someone comes into his area?
Can blindsense be used to target Magic Missile? Fireball? Rays?
One of my players is arguing that indeed, A DD can do all these. Me, I'm thinking people walking by on the street/animals wandering near the campsite might make for a fatigued/exhausted DD, if it's true, but I was hoping to get people's opinions.
Thanks!
Blindsense is worse than blindsigth (see page 561).
He can detect invisible people IF he has line of effect to them. Solid objects block line of effect if the object is big enough. I.e. A roof with small holes blocks line of effect, a roof with holes bigger than 1x1' wouldn't block line of effect. Note that line of effect to the room isn't enough, you need LoF to the individual target.He can't use blindsense with spells that have the "Target" line because "You must be able to see or touch the target, and you must specifically choose that target.".
He can use Effect, Ray, Spread and Area spells.
Note that he still suffer Total Concealment penalties when using Ranged and Melee Touch attacks.
(pag. 214)Edit: ninjaed because I'm too slow!
Us drow are sneaky that way;)
4) Blindsense isn't precise enough for targeting necessarily. It'd let him know what square to target that an invisible creature is hiding in, but said invisible creature still has concealment, so targeting can get difficult. Fireballs don't need targets, that's part of the fun, he just chooses a target square. Consult the concealment rules and line of effect.
Lathiira's other abyss spawned brilliance aside this is really the tootsie roll at the center of the rules-understanding-pop. Dragon Desciples are cool enough without bullseyeing invisible womprats in Beggar's Canyon back home.
Besides it would be much cooler if the character was super paranoid and started to just blow up entire squares with area of effect spells and rolling bouts of ball lightning every time he sensed something sneaking up on him (even if it was a lowly chipmunk or a dastardly chupacabra)
For some cinematic reference I direct you to the scene in Predator where Arnie has slathered on the mud (thus foiling the Predator's "can't see through 20mils of mud vision 2.0.") The Predator knows something is there but can't target Mr. Schwarzenegger with any pinpoint accuracy.