1- If she has an HTC M8/M9 she the phone speakers are plenty loud enough
2- If not, and if you are handy at all this is the answer:
Amazon.com : iSimple IS31 Antenna Bypass FM Modulator for Factory or Aftermarket Car Radios : Vehicle Audio Auxiliary Adapters : Car Electronics
$25. It will plug into your cars radio anteenna (behind the dash) then you run the two wires out to the battery or to the fuse panel. Add in a regular audio cord and you can play your phone, ipod, etc through the car radio ...all in less than $50.
Ive ran the same device for 3 years and used it daily.