Heat lightning, also referred to as silent lightning or summer lightning, is not a thing.
The inaccurate term is often used for the faint flashes of lightning on the horizon or other clouds from distant thunderstorms that do not appear to have the accompanying sounds of thunder.
Even the Farmer'sAalmanac references the phrase.
Well, it's really just lightning that occurs very far away, and the thunder (sound) dissipates before it reaches the observer.
Lightning at night can be visible from up to 150 miles away, especially here in Big Sky Country. Thunder travels a maximum distance of 10-15 miles. The sound travels only this distance because of the properties of air, such as temperature and density, refract the sound up into the atmosphere.
The term heat lightning probably comes from the fact that the effect is most often seen on warm nights of summer and there is an association with sultry temperatures.
So now you can go correct your friends, it's just lightning.