At the back of my friends house his fence is partially fallen over on an angle. The concrete is still attached to the wooden fence post. Could my friend just get some ready mix concrete and pour it into the partially available hole then push the fence back up so it is not on an angle anymore? Is is okay to add new concrete to existing concrete that is years old?

For reference, here is a photo(not my friends) of a fence that has fallen(fully not partially) that still has the concrete attached to it:

http://reliefvolunteers.com/11282008022.jpg