she must not have been paying close attention to her manual... because every single one will have a big circle with a line through it or something similar to indicate you canNOT use it without a shoulderbelt.
Almost 5 and 5 I realize are two different things, but I firmly believe in picking and choosing battles. And here, the easier and more crucial fight is the highback vs. no-back, along with educating her on proper booster use.
You don't need to show her a video... just have her child sit, buckled in the lapbelt, and show her, using the child to demo... the laws of physics... that when the car stops, the body will keep moving, and without a shoulderbelt locking to stop it, the top of the body will move forward... and then just tell her how badly that can injure a spine.
As for the highback vs. no-back thing... I'd start simple and gentle... ask her what happens when the child falls asleep in the car (assuming she does)... because I guarantee you that no 4-6yr old is going to stay upright when sleeping in a backless booster. (well, there are the occasional ones... but a highback is still safer)
if pointing out that doesn't sway her... then talk about side crashes... and show her (using the child to help) how the body will move in a crash... and point out that those eps foam lined headwings (and body wings on some booster models) will help keep her in place and protect her from neck and spine injuries and the need to go see that chiropractor... or from bad injuries or death because she was asleep and not properly positioned in her seat.
IF it's a graco turbobooster... all you have to do is get a model number and a DOM, go online, and she can just order the back (and simply coordinate the prints or add on a cover for the base part so they match)... so she doesn't have to go buy a whole new seat... or point out that it can be put up for another year or two (or sell it on craigslist) and then used as a spare seat.