An i5 costs around $200, and i don't think there are any good P55 motherboards if you have $100. So i think you may have to stretch your budget a little.
i7-920 and x58 motherboard is the best possible combination. But definitely, they would be above $400.
The 920 system + x58 + tri RAM is 52.7% more than i5 + P55 + dual RAM with perfromance gain being within 5% most of the time according to Anandtech.
Hence, I don't suggest him to get i7+x58 unless he has a Microcenter nearby.