Do not argue or admit fault
Keep the reply short, polite and factual. Do not argue the merits, apologise for specifics you have not checked, or speculate about what happened.
Flag it to the person who owns it
Tell the founder, manager or whoever deals with legal matters straight away. Give them the full thread and the customer's account history.
Stop promising things
Pause any refunds, credits or commitments you were about to offer until the owner has decided. A well-meant promise can be read as a position.
Send a short holding reply
Say you have received the message, you are looking into it and a named person will reply by a set time. Do not go into the details of the dispute.
Keep the record clean
Do not delete or edit earlier messages. Add an internal note listing what was said and done, and when. Keep opinions out of the notes.
Decide the rule in advance
Write down who gets told and what the holding reply says before it happens. Then an agent on a quiet Friday does not have to improvise.