2019 Nissan Sentra enviromental controls

My daughter bought her 2019 nissan
sentra in November 2019. Two weeks
ago it started blowing only hot air.
Sometimes the heat will lessen but
comes back. After reading some of the
other questions dealing with this make,
some going all the way back to 2016
model, have told of the same type of
problem.
      She took it to the dealer and they
said that they drove it around for 20
minutes a didn't notice anything. Were
they just "showing her the door", and
when does she have a lemon law
case?