If he supplies you with the skin file, and you put the jpg in the LFS/data/skins folder, and it has exactly the same name as the one he's uploaded to LFSW, then yes, it will work without problems.
btw: You don't need to buy skins - there are hundreds (thousands?) of free skins in the Finished Skins thread - most of the latest skins in there have been uploaded, but you may have problems with some of the older ones - some may still be hosted, some may not - but your friend could re-upload those too, if he were so inclined...