Well we have a found grey and a lost grey, too bad they aren't one and the same but tell your friend to not give up, knock on doors, put up posters and get the word out that one is missing, I will pray that your friend gets the grey back soon.
I wouldn't worry too much about the weather as he can survive thru that, his biggest enemy is predators, tell him to put his cage outside with the door open and some food inside, you never know he might come back to it, stranger things have happened.