Clearing a phone's cache partition typically takes a few seconds, it's a pretty quick process that only involves a singular issue. If his phone is stuck, try to power it off completely and try again -- if it's a phone model with a removable battery, take the battery out to power it completely off, wait a few minutes, and then put it back in; if it's a phone model without a removable battery try holding down the Power button, firmly and continuously until the phone powers itself off.