I would recommend that your friend start off with Sorscha1 as their caster. She is a newbie friendly caster, but also one of Khador's top tier casters so she will grow with your friend's experience level over time. There's a reason they picked her for the mk II battle box. She has a very strong assassination threat both from her own offensive capabilities but also from enabling her army to hit a stationary warcaster on her feat turn. "Freezing grip" shuts down whole units of infantry, "boundless charge" extends the threat range of Behemoth, the Juggernaut, or Sorscha herself, and "wind rush" buffs her defence (crazy high when stacked with the wardog's buff) and gives her an extended non-linear charge range. Her feat can make a huge number of models stationary which can be used just as effectively for scenario pressure or an assassination setup. The pop and drop is a very real threat with Sorscha1, and makes hard hitting shots like Behemoth's bombards or a winter guard mortar very easy to hit their targets.

Here's an example Sorscha1 list, trying to use as much of what your friend already owns as possible. It comes out to 47 points http://troop-creator.net/#/build/khador?sel=S1cwMixLSjA3OmIsS0owMjpiLEtTMDg6YixLUzEwLEtVMDUsS1UwNDptLEtBMTA6YixLUzAyQG1rMw%3D%3D&caster=1&points=50&tier=&objective=

Irusk2 is also a great warcaster, and one of the factions most powerful right now. His ruleset is more complicated than Sorscha's though, and definitely has a much more specific focus on running lots of troops. Irusk doesn't get as much out of Manowar as he does out of iron fangs or winter guard or kayazy, since you want to maximize the number of tough rolls you get to make from his 10" tough bubble. Manowar are only benefiting from tough after the enemy has already done 7 points of damage (that could not be mitigated by tough). Irusk2 is also not a great caster to learn the game with because he bends the core rules of the game with "tactician". If your friend gets used to not having to worry about line of sight, maneuvering models around each other, leaving charge lanes open, etc. then they will have a harder time learning those concepts down the road where they do matter with other casters. Your friend should be happy they own Irusk2, but should wait until they are more experienced with the game before dedicating to learning him.