"Offline" player

I was whispered by a player "YutIngCH" for a trade. I right-clicked his name to whisper him back, but I received a message saying he was offline. He then joined my party, but when I tried to whisper him it still said he was offline, even though he was in my party when I attempted it. Perhaps there is something with his name which messes with the right-click->whisper function?