Hi!
A friend bought a second hand MacBook (legit).
Problem: the former owner can't recall the admin password.
How on earth can I help him wipe the disk clean and install a new OS as this requires the password?
Boot holding cmd+r to get into recovery mode.
From there you can erase & reinstall OS X.
OS X: How to erase and install - Apple Support
If you can't get into recovery mode post back with info on what you see, otherwise it is a case of erasing the Mac & starting afresh.
force erase disk + install new OS without admin password?
