Kudos to your friend for wanting to keep DD Rfing!
Much will depend on the fit and restrictions of the brace:
* How will her joints be affected (Will she still have flexibility or will hip, knee, or ankle be rigid)?
* How much bulk does the brace add?
* Does she need larger shoes to accommodate the brace?
* Can she drape her leg(s) over the side of her existing or a different seat?
* Can she extend her legs up the back of the vehicle seat?
* Can she sit cross-cross applesauce?
Depending on the vehicle and DD's other issues, the brace could make it more difficult to get her in/out of a rfing seat.
The parent will want to look for a seat with a wide seat pan and low sides, a seat known to fit big kids rfing. Radian comes to mind first, but I'm behind the times with new seats. Once the brace has been fitted, a trip to BRU or other B&M store that carries a variety of seats (not just a variety of covers) might be helpful. The majority of stores will let you try a seat in your vehicle to make sure it can be installed or in this case, fit the child.
Worst case, at 2.5 and 33 pounds, I'd be comfortable forward facing, in which case, her MyRide would probably be fine.