Maybe. You'll need to do some work first.
Just some things to know. PowerPC Macs have their drives partitioned as APM. Natively, Windows computers cannot read APM partitioned drives (although Macs can read NTFS volumes).
Additionally, when the new iMac arrives, your friend will find that the drive on this Mac is partitioned as GUID, which is Apple's partition scheme for Intel Macs. Windows based PCs may be able to read that.
When you stick it in a case and connect the drive your PC will tell you it needs to be formatted. So, immediate to your issue before you do any of that install
TransMac.
Having that installed will allow you to access the drive as if it was simply another PC drive.
They give you a 15 day trial so based on how much data you said there was to get off should be plenty of time.
Now, if your friend wants to wait for the iMac to arrive he could simply hook the drive up to his iMac (while it's in the enclosure) and the startup process will allow him to transfer his data from the old drive to the new iMac.
Options. Up to you and your friend.