If you use the computer with the car running, it will not drain the battery at all. Instead, it will be powered by the alternator. If you run the computer with the car off, however, it will drain the battery in a day or so. How long it takes depends on the size of your battery.
Oh, and inverters suck. Use a DC-DC converter.
And it sounds like your friend had something else wrong with his car (faulty alternator/voltage regulator, current leak, etc....).
London, Ontario, Canada
Player: Cyrix 200, 32MB RAM, 10.2Gig Quantum HD, Onboard EtherNet/Sound/Video, Custom Lexan Case, Win95 Kernal w/Custom Player
Car: '86 Mazda RX-7 w/Basic Performance Upgrades