Wow, I CANNOT believe what I am reading! Total Misinformation here!
Firstly, the ball joint is loaded by spring pressure which is considerably high on Mustang front ends. When the tapered stud of the ball joint is locked into the steering Knuckle, the press fit of the ball joint combined with the spring pressure keeps the ball joint secure and in place. Apparently, this guy experienced a breakage between the stud and ball or the ball and outer case of the ball joint wore due to lack of attention. Either one of the above scenarios occurred and the ball joint separated or broke. And possibly when another bump was hit, it knocked the ball joint out of the lower control arm.
Ball joints are designed NOT to fail in normal use and can fail due to hitting something really hard or due to lack of maintenance or service in a timely manner ... that is it! As you may recall, the mustang ball joint is installed through the bottom of the control arm so it is always tensioned serving to keep it in its bore ... hitting a bump would only serve to hold it in its bore even with more tension. And as stated above, the spring applies considerable static tension on the ball joint, even when it is being unloaded after hitting a bump. So, it is virtually impossible for the ball joint to move around or just fall out of its bore. Either the stud would have to break or the ball & socket would have to give out first!
Ford has ALWAYS recommended replacing the control arms for some reason, probably to sell more parts. Though it is nice to have new control arms, they are not necessary but having new bushings and a ball joint in a geometrically correct arm definitely puts things back to factory specs for sure.
I have put about 3 sets of ball joints in my 75 Torino over the last 30 years and never a problem whatsoever. I clean up the bores with crocus cloth and grease the new ones before installing. I then put a block of wood on a floor jack and insert the ball joint from the bottom. Then, I hammer on the lower (outer) control arm right next to the ball joint and they pop right in without too much effort. A good sized hammer is used and the jack underneath serves as a solid base that supports the ball joint as the arm can be pushed (pressed) down over the ball joint.
Most auto stores do have ball joints for the Mustang. But, if for some reason you cannot find them, contact Summitt Racing since I bought a set of Atco Racing ball joints from them last year and put them in my 90 Mustang. As long as you have a press fit, then you are good to go. It should go in freely about 1/4 to 1/5 of the way by hand, and then the friction fit will lock them in there the rest of the way as they are pressed. Been doing it for some 30 years to many many cars and never had a problem whatsoever.
I would bet, if you get a new ball joint, you shall see that it starts to lock up when you install it from the bottom when only 1/4 to 1/5 in the bore ... if it does, your good to go! If or some reason, the ball joint can be pushed into the bore all the way with hand pressure, then you have a problem and then the control arm WOULD have to be replaced. But, I have never seen it or heard of it!
While your at it, you should replace the other ball joint, or you'll find yourself in the same situation shortly thereafter!