Yes, even if the 750Ti can work in a PCIe 3.0 slot, it will also work in a 2.0. 3.0 has twice the bandwidth of 2.0, but rest assured that the 750 Ti won't saturate the 2.0 slot. Your brother won't be penalized because of that. I have a Gigabyte 750Ti on a PCIe 2.0 port in one of my computers, and it's a little beast of a GPU.
When buying the 750Ti, make sure it's a version that does not need an additional PCIe power cord. Most 750Ti don't need it, but a few do. I don't know if the PSU in your brother's PC will have an available power cord, so pick one that doesn't need extra power.