Someone keeps appearing and disappearing from my gchat list?

I have a friend checked as 'auto' on my gchat. They are offline (they have an x next to their name) or are invisible to me. And yet in the span of a few minutes, their name disappears from my chat list and then reappears. This happens frequently, with them being off or on my list for period of time, and I haven't noticed it happen as much or at all for anyone else. It seems fishy -- as though when their name appears they are actually online, but invisible, and when the name disappears they are truly offline. But gchat isn't supposed to be that transparent, you aren't supposed to be able to tell the difference between someone being offline and someone being invisible. Is there some other explanation, or a way, short of installing Pidgin (which I have been unable to install for some reason, I'm on Mac OS X 10.8 if that matters) to find out what's going on?