My fren disappeared from my chat list

[mangib@gmail.com]

I invited and added my fren in the gmail chat list. He accepted but while we were chatting his name all of a sudden disappeared from my chat list and when i tried to send message it says he is offline i cannot send message. I invited, he accepted and I talked to him but while i was talking he again disappeared from my chat list and I have to invite him again to talk to him. I tried that 10 times but still same problem. He says nobody complained but me.This is a very strange problem. Does anybody know what's going on ?