He is 40" and 40 pounds. I know the preschool has his height and weight on file but I don't know if it is passed along for bussing information.
Don't count on it, unfortunately. And most bus companies are fairly clueless and wouldn't necessarily know that he's at the wt (and probably the ht limit too) for a simple inexpensive conventional CR like a Scenera or Cosco HBB.
The best thing she can do is snap a pic of the CR on day one before she puts him in his seat. The driver/monitor will usually play along on day one and let the parent on board even if they don't really want her there.
She can try to contact the bus company before he starts transport but I wouldn't put much stock in whatever they tell her. At the very least, they should be able to tell her if it will be a mini bus or a passenger vehicle like a minivan or SUV that her child will be transported in.
Sorry if I sound jaded but I've had quite a few experiences with bus companies here in NY and honestly, the very best she can hope for is an integrated bus seat. All of our preschool transport buses in my county have them and they're ideal because they are rated to higher weights and you don't have to worry about proper installation:
http://www.safeguardseat.com/products/school-bus-seats/integrated-child-seat/
The next best thing would be a STAR restraint:
http://www.safeguardseat.com/products/star/
HTH!