I bought a iphone 5s but its linked with there account?

My friend just sold me his ipjone 5s (cause he forgot his password and email to it)
but i put it in DFU mode and restored it
it says it was linked with his acct
how do i activate it without his accit

Just factory reset it. Here are some instructions
http://www.digitaltrends.com/mobile/how-to-factory-reset-an-iphone/

Your friend will have to wipe it, otherwise you are stuck.

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