How to get Cydia back after deletion (Accident)?

My friend deleted Cydia on his iPhone 4 and it says he is still jailbroken but he does not know how to get it back.

In this situation, I believe that your friend should do a backup of everything on the phone and then restore the device. Then do the jailbreak again to get cydia.

You may want to have a look at this website regarding the same issue