By budget, just how much is he planning to spend?
I'm sure he's aware of Newegg.com for hardware resources.
He'll find most of what he needs there or he can just buy a Dell HTPC for around a $1000 or less and add what ever he wants to it.
He has a wide choice of BD-ROM drives from the likes of Sony,Pioneer and a few others.
I would suggest he pick up an ASUS XONAR HDAV1.3 DELUXE 7.1 AUDIO VIDEO ENHANCEMENT CARD PCIE HDMI V1.3A DVI. This will take care of his Hi Def video and audio needs.
lagoom.com has them at a reasonable price.
It will allow him 7.1 audio through HDMI and also analog outputs if his receiver or processor doesn't have the new codecs.
A Quad core processor isn't essential, but I would suggest it over dual core if he can swing it.
Tell him don't skimp on the video card. Nvidia based are some of the best IMHO. Get an 8800 series GT card or better for top performance per dollar. The Zotac is very good and cheaper than the competitors.
As Jim states there are several tuner cards out there as well..if he wants to add cable or OTA digital channels.
He has many choices. I would just buy a HTPC bare bones and add the hardware that he wants to it. This seems to be one of the cheapest routes compared to building one from the ground up.
Tell him to get a blue tooth keyboard and mouse as well. This way he can control everything from his lounge chair.
He can go to www.monoprice.com and get all of his cables or accessories.