AMD is a great budget processor, I currently run one in my gaming PC, but AMD does almost nothing better than a comperable Intel does (for a higher price). To interpret the graphs in laymans terms, the i3 would be about twice as good as what he currently has, and the i5 would be about three times better. The i5 upgrade is certainly worth $100, but he should see a significant improvement without it.Hah, I figured it would be, but I have no idea on AMD vs Intel.
So that pretty much confirms the M3 would be superior to what he uses now anyway? Just the i5 would be icing on the cake?
The blue (after looking at it for about 20 seconds) seems to be those things I talked about that AMD actually does better than Intel, but I wouldn't worry about that much, everything else on the i3 definitely makes up for being behind the AMD in a few categories. Most software companies design their software to take advantage of Intel processors first.
So to sum it up, the i3 is overall better than the AMD, and the i5 is far better at almost everything than the AMD.
Of course what GreyFox says is true, it's really depends on how your brother uses his PC, but my best educated guess is that he will see a significant improvement with just the i3, and the i5 would be icing on the cake, as you said.