You could try writing to the old address addressed to the occupier and asking them if they have a forwarding address. Perhaps the local sorting office may have a forwarding address. good luck
If your friends surname is relatively unusual and you know the town of residence you could try the Electoral Roll. By law everyone has to complete this. Councils sell the electoral information to companies for marketing purposes and there are websites where for a fee you can find someone quite easily. It cost my �4.95 to find the address of my wife's lover. Best fiver I ever spent!