What legal recourse does A have, if any?
It mostly depends on whether person A had a reasonable expectation of privacy. The fact that the session was a group therapy suggests that such expectation is not reasonable.
Tennessee is a one-party-consent state, thereby entitling any participant to record the session. Also, your description nowhere indicates that patient B has a duty of confidentiality toward person, A or that the group therapy has a rule that overrides that entitlement.
Under Tennessee law the invasion of privacy is a tort, but it requires the intentional intrusion "upon the solitude or seclusion of another or his private affairs or concerns", Harris v. Horton, 341 S.W.3d 264, 271 (2009) (overruled on other grounds; citing the Restatement (Second) of Torts at 652B). The term "group therapy" seemingly strikes the notion of solitude or seclusion.