Your player's argument is usually now considered flawed. Go back to old D&D; fireball expressly operated in this manner, with text stating that it did fill its total overall volume. So cast it in a hall that's 10' wide, 10' tall and 60' long...the *entire* hall gets filled. Lightning bolt also expressly extended to its full length, including ricocheting. In a situation where, say, the caster is at 1 end of a 30' hallway, and the target at the other, and it's solid rock (which will cause the ricochet) on each end...then a bolt extending to 100' could, IIRC, be started 10' down the hall...it travels 20' to hit the target then the wall, ricochets back 30' through the caster, then ricochets back to hit the original target *again*. What I *don't* recall now is how many times you counted the damage: I am sure that the target takes damage twice, and the caster once (personal immunity didn't exist); BUT it could also have been that the target actually takes the damage *4* times, and the caster *twice*, because they also take the damage during the ricochet. I believe it would actually be the latter...I do vaguely recall, in the old SSI games, *sometimes* being able to nail the critter twice, with the direct bolt and with the ricochet...as long as the caster was far enough away that the ricochet wouldn't hit him.
But, these were the exceptions. Most other area spells had nothing like this. They were also very complex to adjudicate. So by 3E, these notions were discarded. Area spells *don't* fill a fixed volume, so just because you "remove" a section, that doesn't mean that section has to be "added on" elsewhere...it doesn't work that way. Consider this situation:
_______
|m
|.. +
|.... *
|....... m
|
| ............@
m = a monster
@ = caster
* = a solid floor-to-ceiling column, filling the space
_ and | indicate solid walls
The caster uses a burst spell, centering it at the +. The radius is enough to catch both monsters; the column doesn't stop the effect. The *walls*, however, do. The burst would clearly extend well past the monster all the way in the corner, if it can also hit the second monster. But, we don't care about that any more, because there's no more volume-filling.
What I might allow with Spellmaster, is an extension to Modify Spell. While Spellmaster is active, Modify Spell can take any area spell, and make it Shapeable...so it can take on *any* shape desired by the caster, so long as there is continuous line of effect. This could be fairly powerful, but it's also bloody damn expensive. And there might be other options...perhaps using Modify Spell on Spellmaster itself (so now you're burning two 7th level slots) gives Really Good Control, AKA you can completely shape the spell any way you want (within size limits). One would have to test to see what works better. Or something else, that makes sense within the notion that exists now: the spell has a defined area, and standard rules for line of effect apply.
