“I have a tenant that is refusing to pay her rent and is currently five months behind going on six. I had a friend serve her a “Three day pay or quit”.

In speaking with her, she said that it was not a legal service because it was not served by an attorney. Is she right that it was not a legal service, or is the service legal?”

I am unaware of any state which requires that service of process be served by an attorney. Most states only require that the person serving the notice be a) over 18, and b) not a party to the action themselves. That said, it’s not impossible that it’s different in the state where you live. You can easily find out by either calling a local law library, or your local sheriff’s department (sheriff’s often perform service of process).