Take this article as a Precautionary measures.

Step 1 : Pick a Target and start a conversation with him/her.

Conversation :  
You: Hey can u help me really fast?!?
slave: Sure what do u need?
You: I told a friend of mine that i don't have a phone and hes asking whose phone number this is: (Your number here), I told him its urs but he does not believe me…
So can you umm put this as your phone number for like 5 mins??
slave: Ok how?
You: Just go to account settings and on the left it says mobile click on that and put this number in: ( Your number here) and company is: ( Reliance, Airtel etc whatever phone company you use)
slave: K its asking for a confirmation code…
You: (You should get a confirmation code) Ok heres the code: (your code here)
slave: Kk done
You: brb in 5 mins!

Step 2 : Logout or use a different browser and open Facebook , then click on Forgot Password.


Now enter his/her details and click on Search. Then you will get a option of recovering ur Facebook ID with ur email id or Mobile. Click on Mobile. Then click send code, your phone should receive a code , enter that code. Thats it you are DONE!!!

To make sure they dont get their account back i suggest changing the email and deleting any other email registered on the account. 

Its works !!!