If he's got so many 50s as you said, he might just be bored, having experienced most of what the game had to offer, and enjoys passing time helping people, or finding new people to enjoy the game with.

When you have completed most of a mmo, enjoying it again through a less experienced player is a common way to "rekindle the flame". I've seen this kind of thing often in various mmos, and often leads those people to enjoy helping others.

It would also explain why he seemed to be AFK/just standing there while you were away, instead of out playing the game for himself.