Overclocking/undervolting too much can affect effective hash rate. Your friend has probably found good stable clocks so that his effective hash rates are closer to reported.

Try reducing the overclock. Your reported hash rates may come down, but it should get closer to effective. Look for cards that are submitting incorrect shares. If you're using Claymore, you can find this by pressing 's' and it will show you submitted shares per card and incorrect shares per card. If there are cards that have low shares/any incorrect shares, start by underclocking those. I do 10mhz at a time, let it run and do it again like a day later.