What are his options?

I've posted about this before, but have a bit of an update. There is a 5 year old girl at my son's school that is dropped off and picked up by her grandfather. He drives an older truck with a 10 year old Britax seat in it. His other vehicle that he has been using more often lately is a old, small sedan with lap belts only and no head rests in the back. I thought that he puts the girl in the backseat without any kind of seat, but today I happened to park next to him and saw that he had a backless TurboBooster there for her. Besides the fact that it is being used with a lap belt only, it also raises her up so her head is completely above the back of the seat. With her head that high, I wonder if she'd be better off without a booster, but the law in my state requires children to be in a booster to 8 years old. I know neither situation is safe.
I tried to be brave today and confronted him. I just mentioned that those seats are not safe to use with a lap belt only. He got defensive saying that it was all he had for her. I get the impression that he responded that way not because he didn't know or didn't care, but because he couldn't afford anything else. I think that other people have talked to him about that issue (This was the first time I said anything to him about car seats) I tried to tell him that a Maestro might be a good option for her (I was putting a child in the seat as I was talking to him) but he interrupted me and said she was too big for those kinds of seats. He didn't let me explain the higher weight limit and higher harness slots. I just told him that I was just trying to help his granddaughter be safe and he left.
It seems to me that if someone was to help him out and get a safer seat for her, that he would use it. I honestly do not think he can afford to buy anything. I can't afford to buy a seat for her. The 2 seats that I have that she would fit in are used regularly.
What are his options for the child? She is about the same size as my son, so around 40 inches or so, probably around 40 lbs or less. She is 5 years old. The car is really old so I'm sure it doesn't have any tethers.
Here are a few things that I was thinking, but I don't know if they would work or not:
Ride Safer Travel Vest (does it work with a lap only belt and does it require a top tether?)
86-Y Harness (Again-does it require a top tether or some kind of anchor?)
An option that I know will work- a Nautilus or Frontier (but there is no way the grandpa can afford that and I can't afford to buy one for her either.)
Are there any other options for an old car, no tether points, lap belts only, and no headrests?

IIRC, up front in the booster would be better than in the back with a lap-belt only.

Does the front seat have shoulderbelts? I think the best option without having to buy anything is just to move the girl up front, with the seat pushed all the way back.

If buying something, yeah, hwh seat or the ride safer vest. (I like that it doesn't expire. The 86-y harness is only good for 5 years.) I'd rather the girl was up front in the booster than in back without, lap-belt only.

That's an idea worth considering. I'm not sure if the front seats have lap-shoulder belts so I'll have to look next time I see them. I'm sure that the car doesn't have airbags.

I've never played with a Ride Safe Vest so I don't know how they work. Does it require a top tether? Can it be used with just a lap belt?

The Ride Safer Travel Vest requires a top tether anchor when used with a lap-only belt. It doesn't require a top tether anchor when used with a lap-shoulder belt.

Do you think he'll be open to more seat advice? He probably feels he's doing his best and thats all he can do. Honestly, if someone offered to help keep my kid safe, I'd take it

The way he reacted, it seemed like someone had already tried to talk to him about it. It was as if he felt guilty, knowing that his granddaughter isn't safe, but he knows he can't afford another seat. He probably thought that I was going to criticize him or make him feel bad. My goal was just to give him some suggestions, but he didn't allow me a chance to explain. I'm going to talk with a CPST that I know and hopefully he either has a higher weight seat that he can give them or can give some other options. I plan to either write him a letter, so he can "hear" what I have to say without interrupting, or try to talk to him again. If I can't figure out where he can get a free seat, I'm going to suggest putting her in the front seat with the lap shoulder belt, like PP suggested. I don't know what kind of car it is, but seems to have been made in the late 70s early 80s so should have a lap/shoulder belt in the front and no airbags. I really hope that he responds well. I just want him to realize that it isn't my goal to criticize, just to offer help!