The above answer is helpful for telling you what the TL means. By specifically saying "free" TL, Mary seems to be asking for a favor of a future secret. When it comes to girls and gossip, trading secrets is important!
"T. L." is an old abbreviation (originating in the 17th century) that is meant to signify the intent that something is meant to remain a secret. The "T" is the initial for the Latin word "tace," to "be silent." The "L" represents the Latin origin. So, "T. L.," or "Tace is Latin," is a way of conveying a message that is meant to be a secret. Check out the link below for a more historical description.http://www.enotes.com/childrens-hour/q-and-a/what-does-t-l-stand-for-used-by-mary-actii-when-7775