As mentioned, what is his DJ package? I would imagine this would include/require at least a two in two out souncard which are generally external. You could start messing with a multichannel internal card and aiso 4 all but I wouldn't bother.
So
1. Does the setup need to be mobile (laptop or desktop)?
2. What software will be used?
3. What external hardware will be used? Soundcard/controller/mixer etc.
I can give you a start on what I use/have used at home
Set up 1. I5 Imac running ableton live with an APC20 and Touchable (this setup up more for producing as the Imac doesnt have two sound card outs so can't be used for cueing
Set up 2. I5
[email protected] with 8 gb ram. Traktor scratch running using a Traktor control Z2 as the souncard. Ableton running on the same pc using a m-audio connectiv as the the sound card + APC20 and BCR2000
Never had a problem with either in terms of performance but also never tried on lower spec kit.
If you do go laptop my advice woudl be get as high res screen as possible incase he ends up running two software packages side by side