I don't remember ever hearing about her being in contact with them or anything in the game, but she's probably got a basic idea about who they are and what they do - that they're a hacker group that opposes Abstergo. Since she's a techie too, Rebecca's probably got a good idea of how much trouble they cause the Templars.

Check out the Assassin's Creed Wiki (http://assassinscreed.wikia.com/wiki/Assassin%27s_Creed_Wiki) for more info.