1 person (of 4) leaving the lease

I have a house being rented out to 4 individuals. All 4 are on the lease. 1 of the individuals is leaving town and asking for documentation that they are no longer held to the lease. I'm fine with letting this individual out of the lease since the remaining 3 tenants are are fine to continue the lease and pay rent. What type of documentation do I need to provide the person leaving? This individual states that they need proof of not being held to the lease to get a new apartment.