You'll probably have to file or even replace the nut too since the little slots that guide the strings are matched to the strings' thicknesses and you will not be able to mount the E string in the G string slot without doing one or the other. This sort of work is a pain and unless you know exactly what you are doing (which your question suggests is not the case) it is quite easy to wreck it, which may end up costing you money. This idea sounds like more trouble than it is worth to be honest. Is there any reason why your friend can't just buy a left-handed bass?