On the other hand, even though consent may not be required, most counselors will not work with a child if they have been informed that the parent does not want that to happen. You are free to tell the counselor that you do not want your daughter to be a peer helper and to have lunch in her office.
Your objection to this scenario is unclear -- it's obvious that your daughter is considered a leader and a good friend to be chosen to help in this way. Most school counselors do not work with individual students on a long-term basis, so these lunch get-togethers will probably last only a few sessions.
If you have concerns, call the school again and talk directly with the counselor. She will not be able to talk with you about the other child, but she will be able to answer any questions you may have about your daughter's involvement.