Skaven Slaves and Skaven Spells

Yes, he can. Normal rules apply, unless specifically stated otherwise, and Skavenslaves have a rule that does exactly that. Spells are ranged attacks, and the Slaves Expendable special rule (Skaven AB p. 36) specifically states: "Skaven can voluntarily target ranged attacks at enemy units engaged in close combat with Skavenslaves (but no other friendly troops).

But not if a character joins the unit of Slaves, because this FAQ says so:

Skaven Official Update Version 1.7, p.5:
Q: If a character joins a unit of Skavenslaves, can you still shoot at enemy units in combat with the Skavenslave unit? (p36)
A: No.

A ranged attack is any attack that engages targets at a distance. While Shooting attacks are ranged attacks, not all ranged attacks are Shooting attacks. Note also that the fact that the hits of some attacks are distributed following the rules for Shooting attacks do not make these attacks in any way, shape or form Shooting attacks.

 How do we know that spells too can be attacks? Not just because it is rather self-evident for spells that can cause casualties, but because the BRB specifically mentions it on several occasions, e.g.

BRB p. 491:
- The Lore of Fire has its basis in powerful ranged attack spells
- The Lore of Shadow is a force of illusion and deception, relying chiefly on phantasmal attacks to sap the enemy's will (or ability) to fight.

BRB p. 492: All of the spells in the Lore of Fire are Flaming Attacks. 

BRB p. 494: No armour saves are permitted against wounds caused by spells from the Lore of Metal, which also always count as Flaming Attacks. 

BRB Official Update Version 1.9, p. 9:
Q: What are ‘magical attacks’? (p68)
A: All attacks made by spells and magic items are considered to be magical attacks, as are all attacks that are specifically noted as being magical attacks. [...]

Now, it is true that not all spells are attacks, but all spell types can be attacks even Augment spells. If even Augment spells can be attacks, how much more so for uncategorised spells? The spell Wither can  cause casualties - surely, that is an attack? The same applies to the dreaded 13th spell, which even specifies: "No saves of any kind are allowed against the twisting power of the Horned Rat." 

Well, that is the Skaven way and 

Note, however, that unless specifically stated otherwise, all Skaven spells are subject to the general targeting rules.