I've done this 4 years ago and today I need that again.
So after opening main.db file using SQLite Browser you need to:
Determine id of conversation (one person could be in multiple conversations):
select conv_dbid, * from Chats where friendlyname like '%Friendly Name%';
OR you can find desired conversation id using
select * from Conversations where identity like '%accountname%';
Finally, SQL Query to get all messages from desired conversation:
select body_xml, datetime(timestamp, 'unixepoch'), edited_by, edited_timestamp
from Messages where convo_id=YOUR_CONVERSATION_ID;
Unfortunately I discovered that main.db fields body_xml, edited_by and edited_timestamp changed every time person editing/deleting message and there is no backup of body_xml in main.db.
But don't worry! There is a folder chatsync near main.db database (in your skype account folder /AppData/Roaming/Skype/Account_Name_Folder).
All messages in chatsync is in Skype binary format, to read them you can use lightweight free utility http://www.nirsoft.net/utils/skype_log_view.html
Here is the start dialog of SkypeLogView, it automatically selects your skype directory (better close your skype application, but it is not necessary). Choose dates to faster up search process.
Voila!