Currently my home 'workstation' is running a x5670@4Ghz, a 6C12T Westmere xeon from 2010, but it still competitive and can be bought for peanuts today (bought mine for 70£). Its base clock is 2.95Ghz, but it easily overclocks to 4Ghz and beyond.
Anyway, the machine runs an, admittedly overkill, GTX 1070 with a TDP of 150W, so the CPU power usage is the least concern.
Also, since CPU's are idle more of the time than under load the savings by using a newer CPU like Kaby Lake or Ryzen could be interesting. There is also the motherboard. Newer boards consume less power because the chips are on a smaller node.
But I guess the money you saved on buying the CPU will outweigh the electricity costs even in countries where electricity is expensive.
Unless he's running 24/7 and paying way too much, yeah, by a large margin. Even these older Xeons weren't that bad at energy saving.
If you care more about core counts than single thread clock you can find used x58 dual socket workstation MBs which also have support for large amounts of server ECC ram which is very cheap.
Get a second hand workstation on ebay, don't get individual components.