Hi there,
May I ask what your friend mainly needs his PC for?
Unless he uses specific applications that need a hex core CPU, quad channel memory or 40 PCIE lanes then LGA2011 is overkill and really quite expenisve. Therefore LGA1155 is a much better option if he wants a full system upgrade.
The i5 2500K is a great CPU and if his main usage is gaming then it will be plenty for years to come. However, please bear in mind that the next generation of Intel CPUs that will replace the Sandy Bridge chips (including the 2500K) are coming out in the next week or so - therefore if he can wait that long then he could get an even faster and more efficient CPU for about the same money.
That said, if he is mainly gaming then he may want to consider simply replacing his i5 650 dual core CPU with an i5 750 or 760 quad core CPU and then overclock it. These i5 700 series CPUs are relatively cheap to buy these days (~£80), will work with his existing motherboard and are powerful enough to drive even the most intensive modern games. If he goes down this route then he could use the money he would have spent on a brand-new CPU and motherboard on a better graphics card, which will provide a further boost to his gaming performance.