So, a friendly way to do this would just be to add an extra step in between clicking on the user, and opening the chat window, perhaps instead of being
User List > Chat Window
it could go to
User List > Subject/Topic > Chat Window
This would then open that specific chat. Maybe even make it mobile friendly by adding some swipe left loading business to make it all 2014.
The subject/topic part would need a + for new topic, and a back button to go back to chats. The only issue I can see is how the bar on the bottom would handle multiple topics from the same user etc.