Q: my wife forgot their apple id and password.. cannot open their iphone 6

She needs to go and reset it then.  Nobody here can help you - this is a user to user technical support forum.

LOST PASSCODE

You should be able to remove the passcode by restoring the device. Do the following:

Connect the device to the computer to which you normally sync;

Open iTunes (Note: If iTunes prompts you to enter the passcode, try another computer that you have synced with. Otherwise, go to "If you have never synced your device with iTunes" in http://support.apple.com/kb/ht1212 );

Right-click the device in the left column of  iTunes and select Back up (read this article on how to restore http://support.apple.com/kb/HT1766 );

When the backup is complete, select Restore;

Whenever finished, restore from your most recent backup.

To restore read the restore section in this article http://support.apple.com/kb/HT1766

If that doesn’t work you will need to perform a factory restore to remove it.

If you can't remember the AppleID, then you can't use it again i'm sorry. Otherwise, if you can find the mail you have used, you can get the password by entering this site: https://iforgot.apple.com/password/verify/appleid#!&section=password

Hope this can help

I've found this: If you forgot your Apple ID - Apple Support

What does the iPhone say, if you start it? Is it still activated? Otherwise the previous owner needs to send you the passcode/PIN. Did you ever sync the iPhone with your computer?

Also the previous owner needs to kick the iPhone out of its account: http://supportprofile.apple.com

Than you can restore the iPhone from an old backup or set it up new.

If nothing helps: Return to seller and get your money back.