There is StuffPlug-NG2, a plugin for Messenger Plus! 3.40 (beta), that allows people talking offline (actually, they just block you and talk to you that way). Maybe he was using that.
Also, due to server problems, sometimes people appear offline for a long while.
Hope that helped you out.
Edit: by the way, if you are interested in the plugin, you can download it from
the StuffPlug site, this plugin has many more features than that one.
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