Short answer: the largest drive he can afford. 🙂
Long answer: Time Machine only /needs/ enough space to store the actual data on the machine. If your friend's 320GB drive is full, then Time Machine will need at least 320GB of space to back it up. If he's got 200GB free, then Time Machine will need at least 120GB (to back up the actual data).
Then, each hour, Time Machine will need enough space to back up any files that have changed on the drive. How much space this works out to is quite dependent on what sorts of work your friend does and what files are changing regularly.