Subletting to a friend of a friend: setting terms anyway
Meet the person, verify basic references and timing, and explain the room honestly. Then use the same process you would use for a stranger: lease review, required approval, written agreement, deposit receipt, inventory, and house rules.
Avoid letting a mutual friend become the messenger for rent, repairs, or conflict. The subtenant and the person renting the room should communicate directly and keep material decisions in writing.
Clear terms protect the friendship that made the introduction possible. They also prevent a favor from becoming an obligation neither side understood.