Excel 2007 Benchmarks
Excel 2007 Monte Carlo Simulation
Depends on what kind of filters and formulas he uses.
i5-2400 is a good bet if not overclocking (4 core 4 threads)
i7-3770 if more power is needed (4 core 8 threads) , unless you can find the i7 1st gen really
cheap with motherboard (i7 8xx with LGA 1156 or i7 9xx with LGA 1366)