Camera from a available hardware ( i.e CMOS imager main processor board with encoder chips, power supply board, etc) can you even build a camera like a PC?
Actually, you sort of can, but not in exactly the same way.
Companies like Ambarella will often have development kits and can work with you to assemble a prototype unit (provided of course you're likely to commit to something like 10,000 or 100,000 units at some point).
There are other companies in China that do a PC-like build-your-own-camera offering, where you can select components from a menu and they can build you say a 5MP camera with a motorized lens and 24 IR LEDs using a particular imager or chipset.
But really, there is no money to be made trying to bring a camera to market. You can already get a 1080p camera (of potential questionable quality) from China for $15-$20. You can get a *good* 1080p camera for $100. This comes from very high volume, which your friend won't be able to reach.
These days, designing a camera is more about software than hardware. There is potential innovation to be done with web UI's, onboard recording and management, off-site recording/viewing, easy setup and so forth.
I'd encourage him to take the same route Dropcam did in the beginning: write new firmware for an existing camera. Then if the project actually takes off you can explore doing custom hardware.
Companies like Vicon have open-sourced some of their code, and you can use binwalk to "open source" most other camera firmwares. Start with the software, don't worry about hardware because that game has already been won/lost.
