I would respond tastefully. I had this happen several times. Invite them to the office and have a one on one conversation about their issues. I have found at the end of the conversation they really just want to vent and be heard. Hope this helps!
Following! I have a disgruntled owner ( half of my units are pl privately owned) that keeps bad mouthing me and my staff to her new neighbor who is renting. The renter is terrified that she made a horrible decision moving here!
Respond with a genuine comment but don't buckle! Solving a problem doesn't always mean getting a rent credit or free something. You got this! I used to love when unauthorized people would do this and I would call them out passively like 'Oh No I attempted to make contact with you but I show no records of you being a lease holder. Stop by and see me and we can talk about your problems and correct any paperwork issues over a cup of coffee.
As it turns out, the Manager spoke with the lease-holder and apologized for a small mistake (they forgot to change a battery in a smoke detector). The lease holder accepted the apology and had her roommate rove the negative review. But what a pain, grrrr..