From the sounds of it, it seems your friend has done some tinkering with it or rebuilt it and used the case with a new system board. In all honesty, listen to some of the people here. You can't change the board's identification unless you want to seriously invalidate the very warranty you want to preserve here.
Given it is a Dell, they will not 'probe' too much into the board, so long as the Service Tag is valid... You REAL worry is whether or not that Service tag is still within warranty. And even if it is... Unless he actually bothered to register with Dell the transfer of warranty to you, you still aren't going to get much out of it.
You might want to go to Welcome to Dell Support and check the system warranty on its service tag to verify if you should really be worried about its warranty.
My Computer
At a glance
Windows 7 Ultimate x64 and Home Premium x64Intel i7 960 (3.2 GHz Quad Core)12 Gigs (Triple Channel)Alienware OEM nVidia GTX 560 Ti (1.25 Gig)
Computer Manufacturer/Model Number
Alienware Area 51 Desktop and Dell Inspirion 17R (N7010)
OS
Windows 7 Ultimate x64 and Home Premium x64
CPU
Intel i7 960 (3.2 GHz Quad Core)
Motherboard
Alienware Intel based X58
Memory
12 Gigs (Triple Channel)
Graphics Card(s)
Alienware OEM nVidia GTX 560 Ti (1.25 Gig)
Sound Card
Creative Labs X-Fi Titanium
Monitor(s) Displays
Samsung PX2370 LED 23" Monitor
Screen Resolution
1920x1080
Hard Drives
2 320 Gig SATA in Raid 1 Configuration (System/App)
1 1 Tera SATA (Games)
1 1 Tera SATA (Data/Music/Videos)