Without knowing the child's size and behavior/maturity, it's really hard to make a recommendation.
How close to 5 stepping is the child?
What concerns does mom have about the child in a booster. One of my young friends w/autism is a great traveler. Buckle up, hand him a juice cup, and you're good. I know others that are little terrors in the car (scream/kick, pound windows, try to open doors, etc.) or little Houdini's (can get out of any seat, unbuckle, etc.
I would recommend trying the least expensive options 1st: locking the seat belt in a high back booster, an EZ-on Vest or 86-Y. If those don't work, then she could pursue more expensive SN seat options.
Finally, it's been discussed on other threads: obtaining a SN seat through insurance or Medicaid is NOT a short process. She will need documentation and letters of justification of the need for a seat from therapists and doctors. It often takes several attempts and many months to complete the process to obtain durable medical equipment, which is how a SN CR is classified.