She offered corroborating evidence from the get go, and even the 911 dispatcher confirms her story, yet the officer is too busy waving his piece around to get off his power trip.
I'd be mad too. Some element of this volatile combo of deadly weapons and unprofessional police behavior has to change. Preferably both.
Most cops go their whole lives without firing their gun--adding an extra gun rarely makes a situation better. While TV and movies like to portray heroic feats of gunplay, in reality, 90+% of the bullets go errant in situations where a gun was actually useful and required.
Police in the US are generally seen as adversarial, while police in the UK are generally seen as mostly helpful. In my estimation, the problem in the US is one of culture. Police here are looking for a reason to charge you with a crime, not trying to keep you out of trouble. It's "us versus them" on both sides.
And yet you wrote in another comment that had you been the son-in-law, you would've brought your rifle to go talk to the person at the car. Why? In your scenario, you already called 911, what's the point of brandishing the weapon if you're not prepared to shoot someone for possibly stealing a car?
It's not unreasonable to expect that someone willing to steal a car might also be willing to harm you to do so.
Also, if someone feels like the expected penalties for stealing a car and wounding/killing someone are close enough that they should be prepared to kill someone while stealing the car if needed, that speaks to a failure of the justice system: either the penalty for causing physical harm should be much higher, or the penalty for stealing a car should be much lower.