On 6 March 1944, Frey was killed over Sleen, the Netherlands after attacking a formation of Boeing B-17 Flying Fortress bombers.

During his career he was credited with 32 aerial victories including 25 four-engine bombers, all on the Western Front.

(J)—1st fighter group) of Lehrgeschwader 2 (LG 2—2nd Demonstration Wing), an operational training unit tasked with the evaluation of new types of aircraft and tactics.

[3] World War II in Europe began on Friday, 1 September 1939, when German forces invaded Poland.

The next day, Frey claimed his first aerial victory when he shot down a Polish PZL P.24 fighter aircraft.

[9] During this aerial combat, Frey was shot down and killed in action in his Focke-Wulf Fw 190 A-7 near Sleen, 30 kilometers (19 miles) west of Meppen.

[13] According to Weal, Frey, together with Major Hermann Staiger, was the most successful fighter pilot against the heavy bombers while flying the Bf 109.