I think it depends on your relationship with the neighbor. If the backless booster is being used for short, infrequent trips, it is probably adequate. It wouldn't be what I would choose, but I'm frankly glad she's using a booster at all. If the child is going on longer trips, a highback booster offers some really serious benefits especially when the child falls asleep in the car. If it wouldn't be financially difficult for her, you might suggest something like a Graco Turbobooster which easily transitions between a highback and a backless booster. They can pop the backless portion in the vehicle for those quick trips around town, but they can put the back on it if they're going on a trip or going somewhere (i.e. the zoo or the beach) where the child is likely to be exhausted for the ride home and need the extra support to stay in place.
But if you're not close with your neighbor... frankly, I'd probably leave it alone. A lot of parents who don't own cars just skip the booster entirely. I have to give her some credit for at least having the backless booster.