2014 INFINITI QX80 Steering Problems

My daughter was driving this car home from college on I80 east from Iowa to joliet and almost was involved in a accident. The stearing started to lean to the left and when she had to slow down and brake the stearing went hard to her left until she released the brake. she pulled over and called me i came out and drove it home slowly. i took it to my mechanic which i thought it was an alignment issue or a caliper on rear brakes. However it was brought to my attention by the mechanic that the left side of the rear sub frame was rootted out by rust and broke almost causing a major accident. I have been to the Infinity of Naperville three times this year and muliple times over the course of 2yrs for maintence and the rear sub frame passed the inspection everytime! This should...