Either card would work, but do not expect a huge boost in performance until you hit the $100+ range. These are both very entry level cards, and nither will game well... however, nither requires extra power either, so if your PSU load is of concern then these are not bad.
Of the two I think I would swing for the 520... but it check benchmarks for the specific games you intend to play.
Either card would work, and be a nice boost over the integrated graphics.
On a price/performance basis, you will get what you pay for. If gaming is the objective, buy the fastest card he is comfortable paying for.
But, before upgrading the graphics card, he needs to increase the ram past 1gb up to 3gb at least.