My friend gave me his iPhone 6, but he forgot the password.

Your friend will need to somehow remember his password. Then, he/she will need to follow these instructions:

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

The phone is completely useless otherwise.

Hello myfob,

If the device passcode has been forgotten - meaning you cannot get into the device, the phone can be erased using a computer.

-> If you forgot your iPhone passcode - Apple Support

However, after resetting the device it will promp you to sign into whatever iCloud account was used on the device. Without this information the phone cannot be used. This is called an Activation Lock.

If this password or account is unknown, try the following steps to recover or reset the iCloud password:

Reset Password -> https://iforgot.apple.com/password/verify/appleid

Forgot Apple ID -> https://iforgot.apple.com/appleid

If those did not help, Contact Apple Support:

-> Contact Apple for help with Apple ID account security - Apple Support