Assuming that they are having a direct 1-1 message (and not chatting in a public/private room), nobody but those two people will ever see the message history. If you're in HipChat Cloud, even administrators will not be able to pull the history up in an export.
Administrators of HipChat Server have the capability of retrieving 1-1 messages, but it's not as simple as just going and looking at history in the web interface.
Hope that helps!
...have mention name FirstnameLastname. Fe.: @MaartenCautreels This certainly helps to know who is being mentioned in a conversation. Imagine your colleague sends the following message...
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