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Some more info is needed. You mention rear brakes only but what about the fronts in all of this? Squeaks and squeals can be a common problem with disk brakes.
So called generic rotors and pads are fine for the most part so that really should not be an issue.
If the complaint is squeaking on the rears only and this happened with the first replacement set of pads and rotors it could have been due to an installation fault. (there are several ways of mucking this up or having a squeal return)
It’s also quite possible that driving habits and conditions could be behind this. (Hard stopping after jackrabbit starts, etc.)
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If the rear rotors and pads were replaced 2 months ago and they’re worn that badly after such a short period of time could mean a number of things.
A park brake cable hanging up.
A hydraulic problem causing residual pressure to be maintained on the pads.
Sticking caliper slides.
While I have no idea if any of those is the cause of this problem, inspecting or servicing these things should be part of a proper rear brake job.
At this point I’m kind of leaning towards one of the above possible causes and your fiancee not being at fault. Depressing the brake pedal to shift the transmission should not cause the rear brake wear. However, cars can be downright goofy sometimes and one should never rule out some oddball glitch. This shift problem is likely due to an interlock fault. This should not be difficult to repair.
I’d like to add to OK4450’s suggestions
inoperative front brakes (due perhaps to stuck calipers or collapsed lines)
and perhaps a proportioning valve (see below link).
You need to get this looked at by a competent shop. The good news is that diagnosing the problem is not rocket science and the fix won’t be cost prohibitive.