Brakes lock up on 2016 Dodge Ram

We just bought this dodge about 2 months ago.  
We had a 3" AEV lift kit put on by the dodge dealer,
a lift lit.specifically made for the dodge ram truck.  
We have driven it a total of about 10 days and it
has about 700 miles on it, approximately 400 miles
are highway miles. The remaining time it has been
parked in the garage.   2 days ago brakes locked
up in the parking lot and it was towed to our local
dodge.  The mechanic stated the brake fluid was
contaminated which caused brakes to lock up and
needs all new braking system to a tune of
$4000.00!.  The truck has not been anywhere
where it could be contaminated and has not had
any service done (again only has about 700 miles).  
Local dodge dealer called dealer we purchased
truck from and they stated they never touched the
brake lines during lift kit install.  However, when
we read online about how to install an aev lift kit it
states you have to remove the front brake lines.  
Being that we have about 400 highway miles with
not a.lot of brake use on highway and only brake
usage would be locally in town recently, how long
would it take for contaminated brake fluid to cause
the brakes to lock up?  I am concerned that with
the brake lines being removed and bled for.lift kit
installation that the dealer topped of the brake
fluid with some other fluid that was not brake fluid.
And with not a lot of driving and brake usage I just
don't know how long it would take for the whole
system to become contaminated causing the
brakes to lock up?   Thank you very much for your
time.