Vent /question Explaining a seat isn't right

My niece is 3years and 4months, ff, with us in a Nautulius and used to have a cosco as a secondary seat but she had a growth spurt(litterally went from a 4t to a 5t shirt in six weeks) and now doesn't fit it.
She goes to her fathers every other weekend, and about a month ago my sister noticed that in addition to extreamly loose straps, her ears went to the top of the seat. She explained to him that he needed to have a new seat the next time he took her. When he dropped her off, she was in the same seat.
The next time she went with him my sister had me make a list of seats that'll last, comparable to our seat with prices and she explained them to him. Yet again, he had the same seat. But this time told my sister he doesn't have the money for a new seat so she offered to let him borrow her seat next time.
Friday I droped her off with him, and when I took the seat out for him to use he refused telling me that the seat he had was perfectly safe and when I explained that if he read the instructions his seat would say the childs shoulders cannot go over the straps he said thats a matter of my opinion and that he looked at the seats on the list, that shes too small weight wise for any(she is 37lbs!) And then said he has a booster he could use for her

Could someone please give us a way to help him understand why it is unsafe for him to be using the seat he has or a booster for her. The law doesn't say she must be in a harness, but he seems to be over looking common sense in order to pinch pennys and be stubbon.
I know it would do no good talking to his mother or girlfriend as his mother puts my niece in the front seat of her car(my sister and her have had many arguments about that) and his girlfriends child is six and not in any type of seat, riding in the front of his truck!

Wow, what a tough situation!

The first thing that I like to point out as the difference between a 5 point harness a booster, is that the harness doesn't allow the child to move out of position. So, if the child gets wiggly, as many 3-4 year olds do, they can't endanger themselves because the harness holds them into place. But when you put a 3-4 year old in a booster, now they become in charge of their own safety because they have to sit properly for the seatbelt to work. If the child is wiggling and pulling the seatbelt loose and leaning out of position, it's not going to work correctly to keep them in plane in an accident.

There have also been a few threads on here about getting proper car seat usage into custody agreements (not sure if that's exactly the proper term), etc. My skills with the search feature are deficient, but I'll see what I can find. If not, maybe someone else remembers what threads I'm talking about.

I'm not one to pull out the scary YouTube videos willy-nilly, but in this case (considering moving a 3 year old to a booster), I'd show him Belle's Gift. It is a video about a 3 year old who was in a crash and died because she was in a booster and her body wasn't old enough for it yet. (The older sister, also in a booster, survived the accident).

What state are you in? Many states' laws have "proper use" wording, which would preclude her from riding in a forward-facing harnessed seat whose straps are below her shoulders.

[ame="http://www.youtube.com/watch?v=y2LFo8vVi04"]Belle's gift video[/ame]

We're in Washington. For between two and four the law says they must be properly restrained in a forward facing child restraint.

For putting it in the parenting plan, when the issues started we looked in to what was in there and one copy, my sister had corrected it to say that she must be in a proper five point harness seat and his lawyer had added until school age(she will not be out of a 5point harness at five with my sister). In the final copy when we looked at it it just says father must have a carseat. He had taken out anything about proper use or harnessing.

I would LOVE to show him the video, but I know he won't listen. When we try to talk to him about anything he won't listen.

I just found the laws from this year, and it says "the restrain system must be used correctly according to the car seat AND vehicle manufacturer's instructions"