First of all. Why Windows 10? If she is happy with Win 7 why change?
Secondly, do you live in Fethiye? If you do I can probably help, but over long distance it becomes more difficult.
By the sounds of the present partition set up, it may have had XP on originally, so there could be other issues which can't be discussed here. A likely 40GB drive is quite small for an original Win 7 machine.
Normally Windows (which may still be XP, Vista etc) is installed on drive C: by default. You say Windows is installed on another unnamed 10GB drive. The other drive may contain system files.
Click on (start) then right click on (computer) left click on (manage) when next box appears left click on (disk management) you will then see all partitions including their drive letters and names on the hard drive of the computer, including any hidden ones.
It is possible to resize partitions here, but great care is needed. It really is best left to an expert.
To be honest, if you are not local, there is not much else I can do.
If in doubt, start on line one again. My own opinion is stick with 7.
Rick