If your friend has an AT&T phone and she is texting another AT&T number using a text free app, can the person receiving the messages from the the text free app call AT&T and find out who the person is and any previous conversation the person is having pertaining to them with another number?
1 - Can the person receiving the texts call AT&T to track down who is texting them?
Answer - Probably not. If they are using an app to send text messages then the txt messages will show a source of that Free App application vs. the senders mobile number. Does the Free App not forward the Mobile # that is sending the messages?
You might be able to contact the Free App Customer Service and check with them however.
2 - I don't fully understand the second part of your question however I think you are asking if AT&T can help track them down and give you information about who else the Source is texting.
Answer - I highly doubt it. I cannot imagine AT&T giving you any information other than what they might have for your phone only. Anything additional regarding the Source could be a privacy violation.