I got for free a used Iphone 5, but the old owner left the ID on the phone

The only one that can remove the iCloud lock is the previous owner of the phone. If you cannot contact them and get them to remove the lock, then the device is useless to you. Activation Lock cannot be bypassed, and Apple will not help you since you are not the original owner. Turn off Find My iPhone Activation Lock - Apple Support

As noted, you cannot bypass the activation lock. If the prior owner or their family members are not available to remove the lock, there is nothing you can do at this point. Since you said you got it free, you are not out any money. However, you now have a brick that looks like a phone.