There is a commercial product I've worked with, NSI DoubleTake, which is used on servers. After an initial Mirror, it switches to a Replication mode that immediately copies any changes over = near real-time, depending on how fast the network is. You can define filters on what to replicate, and control bandwidth and schedules. However, it's not really "backup", because if there's any damage on the source (e.g. you delete a file)... it replicates the damage too. Unless you limit transmission times, that happens Immediately.
You don't say what version of Windows your friend has, but if it's Vista, he has the
Shadow Copy option built-in. This is a "snapshot" technology adapted from Windows Server 2003, that "copies out" modified data blocks to a new location quickly and transparently. What you get is a friendly "Previous Versions" tab on a directory or file's Properties that lets you roll back changes, which sounds like it fits the bill. However, without going into too much detail about snapshots, note that it copies only the disk
blocks that change, not whole files or directories, so it relies on the original drive being up: it's useful, but it's not a full replacement for backup to an external location.
What about the standard Windows Backup program? Select the external drive as the target, Advanced mode in the Wizard, set up the schedule you want, and use Incremental mode to back up only changed files.
Heck, if your friend wants to get back to basics, use XCOPY /S with /M or /A (using the file Archive bits).
