It should be marked as seen only if the other chat participants other than the sender sees it.
I test this with a raw JomSocial site. Using phpmyadmin to monitor the table chat_activity, I find that
1) when a message is sent to chat A, the last action of chat A is "sent" own by the sender;
2) if the sender now visits chat B and re-visits chat A, chat A's last action will be "seen" own by the SENDER
Hence, chat A's last action will be wrongfully set to "seen", despite the fact the other participants haven't actually seen it. This also affects the notification emails that are supposed to be sent to inactive users who's got some unseen chat messages.