Cougar will redirect the Vault Dweller to this man if talked to. Flash has a real bad attitude and the only dialogue he offers is telling if the Doc is around.
If the Vault Dweller loiters around Doc Morbid's hospital between 6 pm and 6 am, Flash will initiate conversation and order them to leave. Responding "eat my dung" or remaining in the area after Flash's warning will cause him to become hostile and attack the Vault Dweller. Actually being in the hospital building is not necessary to provoke Flash, it can occur when outside and in his field of view through the doorway.