The fact that the two drives have different operating systems should not present a problem.
Now, let me see if I have got this right.
Your friend has a Pc which originally had a bootable hard drive with Windows XP Pro. You then fitted a new hard drive, presumably with a pre-installed operating system of XP Home edition. The original drive was subsequently connected as a primary slave drive and the new one as primary master. The PC will now only boot to the new drive,. even though the system bios recognizes both.
This seems to point to either a jumper problem on the old drive or a boot.ini problem.
As previously mentioned, If you friend is able to boot to the new hard drive, he/she, should still be able to access the contents of the old drive, which should be drive D.
I need to know the name of the windows folder on both drives and the contents of the boot.ini file on both.