From what you say, I assume each of you pays your own rent separately direct to the landlord (or his agent if there is one), and that each of you paid your own deposit to the landlord.
If that is the case, it does not seem to be your responsibility to do anything about getting someone to replace the person who has left. That person should have given the necessary notice to the landlord (probably one month to expire on a rent day) and it is then down to the landlord to re-let the room. If the person did not give notice, the landlord could chase them for outstanding rent.
I suggest you make sure the landlord is aware that the person has gone.