I have a friend sitting far away from my home. He is not a computer expert
but wants to follow my advise to split his (only) current partition (C
into two partitions (C: and D.

I would do such an operation from a bootable CD with Partition magic
or Paragon Partition Manager.

Howver as I said before I am NOT sitting in front of this computer and he
can not do such operations like choosing extend partition and create logical and ...

So I am searching for a way to tell his computer (through remote session)
from a running session to do the patition resize+creation at next startup/boot.

In detail the following should be done:

1.) Shrink partition C: from 80 GB to 15 GB
2.) Create extended partition in the new unassigned area
3.) Create logical partition D: in thejust created extended partition
4.) format D: with NTFS

Can I do all this from a running WinXP?

I could imagine that there is a built-in command program which
uses a script at next boot time to perform all these operations.