Sometimes when my DD gets fussy in the car I turn on a childrens CD for her. It distracts her and calms her down about 95% of the time. A mixture of happy, fun music with calming nursery rhymes or lullabys works best.
Also, this sounds really blunt and almost mean to say to anyone, but with some people being blunt is just what gets through to them. I wouldn't say this to somoene I'm not close to but I've told friends who turned too early... whether the excuse is so that they can see the baby or the baby screams...
By turning them forward facing they are taking a risk so that they can see the baby for the length of the car ride, or baby will not scream for the length of a car ride, but they are risking NOT seeing that baby for the rest of their (parents) lives. They are risking NOT hearing that baby cry or scream for the rest of their lives... coming from a parent who has been in a crash WITH my DD in the car, her cry that day was the BEST sound I have ever heard. Better than the day she was born, because I knew she was alive, even though I couldn't see her ( and FWIW- we had also been listening to a baby CD for 6 hrs that day on a road trip, just me and DD, to keep her calm.)
Good Luck, I hope you can get through to her.