Ok, I wasn't really sure from what you wrote, but I'm assuming to start with you don't want the lines from the paper. There's no good way to remove them (textured lines on textured background intersecting with other textured lines you want to keep), so I've had a quick stab at it for you:
From here on in it's really simple to colour the image whilst retaining these lines. All you need to do after opening the above image in ps is:
1) select all (ctrl+a)
2) copy (ctrl+c)
3) paste as new layer (ctrl+v), set layer type (the dropdown menu under the word "layers" in the layer window (f7 if it isn't open already)) as any of:
darken
mutliply
colour burn
linear burn
All give slightly different results depending on colour/tone values of the respective layers, but will give you roughly what you're looking for (darker parts of the upper layer show, lighter parts are transparent). Multiply tends to give a smoother effect as midtones pick up the colour of the background layer, whereas with darken greys remain grey until they're lighter than the background, at which point they become transparent.
4) blank the background (select background layer, ctrl+a, fill (g) with white)
5) create new layer (shift+ctrl+n) <- this will become your "colour" layer, you can create as many of these as you like
6) drag the layer (2) underneath the layer that has your drawing (1) from the layers window.
7) paint (or whatever) onto the colour layer (layer 2)
8) you may well want to adjust the brightness contrast (alt->i->a->c) on the drawing layer, to make the drawn layer "cleaner", but that's all personal choice.
The figure on the left is set as "multiply", the figure on the right is "darken". As you can see the midtones pick up the background colour on the lhs, whereas on the right they stay the grey of the original image.
If your friend is doing more of these drawings, and the lined paper isn't a crucial part of the artistic concept, i'd strongly recommend working on blank paper.