The ball joint stud is tapered and if the break point was below the knuckle it would indicate that over tightening was not the likely cause I would think. And if that is the case the ball joint was defective.
I replaced one ball joint due to the nut coming off and questioned how that could happen. All other failures were the result of wear due to lack of grease. Have you seen a failure similar to the OP’s, @Tester?
That’s the exact point the ball joint broke that I was installing.
Imagine my surprise!
Tester
Strictly my opinions:
If I agreed with the idea that the part manufacturer is only obligated to replace a defective part as outlined in their warranty, I also believe that a customer can reasonably expect a certified mechanic will use parts that are suitable for a proper repair and for those parts to be installed properly.
If aftermarket parts were chosen to enhance the profit/convenience of the shop/mechanic (and especially if priced out at OEM prices, as is sometimes done), then the mechanic/shop has to be responsible for the consequences, all of them.
Somehow a determination needs to made as to what likely happened. If a car is repaired properly using proper parts and an out of the ordinary impact results in part/parts breakage then that’s on the owner/operator of the vehicle.
If evidence shows that the damn thing just broke or fell apart then that’s on the shop/mechanic and it’s the shop/mechanic to get a part/parts replaced under warranty if they consider trying them again, but that’s their little problem.
Unfortunately the driver needs wheels, ASAP. If the repair facility doesn’t own the problem and remedy then it is probably going to take some time to get to a remedy.
If that happens, I believe most states have a way of reporting complaints pertaining to licensed mechanics and shops and a way of going to arbitration over a matter. Threat of this chain of events, if needed, could possibly help get something going with shop/mechanic.
CSA
CSA
I got the new control arms on before this guy told me i needed ball joints in dec.
I’ve been an auto mechanic for over fifty years.
And I’ve had my share of replacement parts failing after installation due to a manufacturing defect.
Unless you hire a lawyer and sue the manufacturer of the defective part, you will receive no compensation for labor or property damage from the part manufacturer.
All you get is a replacement part.
Tester
That’s crazy. We’re still waiting to see what the assessing shop will say. I can’t believe that I’ll have to likely eat the damages. Im the only one who’s fault this wasn’t… I most likely will be out the car and money because i just put all my money into it.
I agree and as I stated in my opinion piece, in this case, that’s for the mechanic/shop to screw with. But, that’s her/his problem, not the customer’s concern.
CSA
But once again the need for car owners to find an honest and reputable shop and become a loyal customer is evident.
I woujld hate to even hazard a guess on what happened as there has been multiple repairs which could involve the control arms including the CV axles.
Just some food for thought, but if during an ensuing repairs the ball joint was separated to replace the CV axle and the ball joint nut was not tightened securely the looseness could have caused it to snap.
When I blow up the picture, it’s too blurry to make that evident. I can’t tell if the stud is there, or if I’m seeing the side of the hole. That’s why I said in my 1st comment that it would be nice to see the knuckle.
A pickle fork on an air hammer could damage a ball joint.
https://www.amazon.com/OTC-4559-Manual-Pneumatic-Pickle/dp/B000F5HUTO
Get your third eye checked 
Why would a pickle fork be used to install a NEW ball joint?
Tester
It might be used to separate the “new” ball joint on the second repair which replaced the axle.
I’m third eye blind. 
A pickle fork will damage the boot on the new ball joint.
You use something like this instead so that doesn’t happen.
Tester
Note I said it might be used, not should. 
And big box stores aren’t helping educate people as to the proper tool:
I got my answers today. The ball joint was not put on properly and when it broke it also broke my half shaft (i just replaced months ago) and pulled a brake line. It’s about $300 to fix. I’m taking the bill to him with a statement from the other shop saying what went wrong. The shop it’s at now backs all their work. They even talked advanced auto to extend the warranty on the half shaft. Hopefully this guy will save himself a lot of bad reviews and small claims court by paying me for the damages.
Don’t hold your breath.
Auto repair shops aren’t ones that willingly admit they messed up a repair.
Tester
make sure they not only get your car on the wheels, but repair any body rust protection this incident might have damaged: I definitely see fender protection liner sticking out, not sure if wheel rubbed off any paint/undercoating. you do not want your car to start rusting out in this area from this accident
I’m 100% sure wheel alignment will be badly skewed and needs to be done at their cost too
they have to make you whole, so do not settle on $300, which will probably cover only under half of the costs