O.C.G.A. 17-4-60 (2010)
17-4-60. Grounds for arrest
A private person may arrest an offender if the offense is committed in his presence or within his immediate knowledge. If the offense is a felony and the offender is escaping or attempting to escape, a private person may arrest him upon reasonable and probable grounds of suspicion.
GA Code § 16-7-1 (2014)
(b) A person commits the offense of burglary in the first degree when, without authority and with the intent to commit a felony or theft therein, he or she enters or remains within an occupied, unoccupied, or vacant dwelling house of another or any building, vehicle, railroad car, watercraft, aircraft, or other such structure designed for use as the dwelling of another. A person who commits the offense of burglary in the first degree shall be guilty of a felony
The key phrase:
“If the offense is a felony”
Trespass in Georgia is a misdemeanor. There is no evidence that Arbery committed burglary that day.