'Chrisley Knows Best' Cast Member Threatens Suicide, Savannah and Todd Rush to His Aid

"Chrisley Knows Best" cast member Nic Kerdiles -- who at one point was engaged to Savannah Chrisley -- is alive today, because she and Todd Chrisley sprung into action as Nic was having an alleged mental breakdown ... while brandishing a gun.

The father and daughter can be heard on a panicked 911 call, obtained by TMZ, made Monday night outside a home in Nashville. Law enforcement sources tell us the call came in for a suicidal man, who had a gun to his head.

Todd and Savannah believed Nic was in the grips of a psychotic episode.

You can hear Savannah screaming in the background of the 911 call -- made by a friend on scene -- as she pleads, "Please, no!" You can also hear someone, who we believe is Todd, yelling, "Look at what you're doing to her!"

The caller tells the 911 operator Savannah was able to smash a window to get inside to her ex-boyfriend, but cut her hand in the process. Eventually, the Chrisleys were able to get the gun away from Nic ... and he agreed to sit in Todd's Range Rover until cops arrived.

You can hear the caller indicate Savannah's ex needed to get some help in treatment, and while cops were unable to tell us what happened once on scene ... there's no indication he was arrested.

Nic famously proposed to Savannah on an episode of "Chrisley Knows Best" a few years ago, but the relationship eventually came to an end.