No, he's not ok in the booster. A mature booster age is 5-6 years old.
She has a few options. Is he long in the torso or short? If he's short in the torso an Apex 65 would work for her. It's $110 (
http://www.albeebaby.com/sa1stap65boc1.html), has a 65 pound harness with 17.5" top slots and then turns into a booster to 100 pounds. It require head support from the car to the level of the child's eyes, though.
For $150 she can get the Graco Nautilus (
http://www.walmart.com/catalog/prod...00000003260330&ci_src=14110944&ci_sku=8112222). It has 19" top slots, a 65 pound harness limit, and then turns into a high back then a low back booster to 100 pounds.
For $180 she can get a Sunshine Kids Radian with a 65 pound harness and 18" top slots.
Those are it for higher weight seats under $200. Of the three I'd go for the Nautilus.
For less mainstream approaches, she can look at the Ride Safer Travel Vest (
http://www.amazon.com/s/ref=nb_sb_n...&field-keywords=ride+safe+travel+vest&x=0&y=0) for $100. The large will fit him for quite a while. There's little side impact protection, though, and it requires a top tether with a lap belt.
She can use an EZ-On harness. (
http://www.ezonpro.com/products/familyVehicle/86yHarness.shtml) I think it's about $100, but I'm not at all sure. It can be used up to 168 pounds and it can be used with a high back booster. However, it doesn't have a crotch strap, so it's just a four point harness.
Again, of those I'd go for the Nautilus. If $150 is completely cost prohibitive I'd probably look at the Ride Safer Travel Vest/EZ-On, then the Radian and then the Apex.
Wendy