MiKTeX doesn't run on Mac OS X, so probably your friend has installed MacTeX, which by default includes the CMSuper font package which provides Type1 renderings of the European Modern fonts that are used in documents that have
that's recommended when writing in French or another European language.
The fact is that the basic installation of MiKTeX doesn't include this font package
(see Why are Bitmap-Fonts used automatically?). Install it with MiKTeX's package manager, otherwise the machine running MiKTeX will display bitmap fonts created at 600dpi, which is the most probable cause of the difference you see.
You can avoid this installation by adding to your document
that makes LaTeX use a different set of fonts that, however, are almost indistinguishable from the Computer/European Modern fonts. See Latin Modern vs cm-super? for more information.
Just to show what happens
Image with CMSuper fonts disabled
Image with CMSuper fonts enabled
Image with the Latin Modern fonts (