I've previously opted for Eibachs in 2 of my last e60s.
I went with the Eibach ProKit for my 2006 e60 520d...very subtle drop and not much difference from standard spring, in regards to drop and comfort.

I opted for the Eibach Sportlines in my 2007 e60 520d. The drop with these was that bit more, gave the car that more road presence without losing its comfort.

I'd recommend either...depending on how much of a drop he wants.