My question involves criminal law for the state of: Florida

Upon receiving a court subpoena for a murder trail, I have a relative who is bipolar and has gone into a total meltdown. This episode includes paranoid illusions that "They", who ever "they" are are out to get him and his entire family is in danger. This meltdown has even included a suicide attempt. They have a doctor who can support the medical side of things.

Under Florida Statutes 90.804 - Hearsay exceptions; declarant unavailable. Can they be exempt from the subpoena?

They were not a witness to the actual murder, was no where in the area and is only wanted as a supporting witness to the killers previous lifestyle.