Yikes! That's some alarming advice, even in a special needs situation where rules might need to be ignored, when there are perfectly acceptable options which don't need to bend the rules. Glad your friend is realizing that something needs to change.
How tall is the child? He is definitely light enough that she has her choice of seats. Depending on how tight her budget is exactly (and count in the fact that she may be able to sell her FF seat to help offset her cost), there are a couple options which might be better than others. I am not sure how much the low tone affects his ability to sit upright without slumping sideways, but I am going on the assumption that this might be a factor in seat choices.
While the Radian would last RF a very long time, it may be out of her price range and I am not sure how supportive it is, exactly. Don't know if there are any limits on using the comfy soft body pillows or how well they work.
I know the Graco MyRide is very good for providing support to slumpy kids, and it has a 40 lb RF limit, but the shell is not as tall as some others so if the child is tall it may not be the longest lasting option. It's about $220 or so. Oh and a bonus with this seat is that installs very reclined once it goes FF compared to most seats, so it may help prevent headslump once he is FF as well. Great choice IMO if he is slight overall.
The Safety 1st Complete Air looks like it would be fairly supportive as well. I looked at one briefly today IRL for the first time, and it seems to me it was pretty enclosed. It is much taller and also has a 40 lb RF limit. It was about $270??
The First Years True Fit RF to 35 lbs and has a nice tall shell but it has a wide interior and widely spaced straps which seem to cause some issues with potentially trying to slip off the shoulder, especially if the child tends to slump to the side at all. It is about $199, or often $150 on sale at Zellers. While it offers less support, she could always pad his sides with rolled receiving blankets, and just being RF and more reclined should eliminate a lot of slump anyway.
For some more budget friendly options there is the Cosco Scenera (~$90-100 not on sale) or Safety 1st Avenue (~$150). These seats RF to 35 lbs and have decently tall shells for a budget-friendly seat. They would not last very long FF compared to the other options. I don't think they provide as much support to a slumpy posture (at least not the Scenera) as the MyRide and Complete Air but if it's what she can afford, then they should work, same reasoning as the TF that just the fact that the child is reclined when RF should be a great help as it is.
I think a good option would be for her to take him in to the store and try sitting him in some of the seats she is considering to check for fit and comfort and she will be able to see which ones look like they would work better. I went by my Zellers today and was THRILLED to see the CA, MR, 2 models of TF (no ARB), the 35lb models of AO (another ok option but not as popular as a choice for a variety of reasons - but she could look at that too as long as she promises not to use it in booster mode
) and some awesome choices for FF as well. Some Walmarts apparently also carry the MR and CA as well as the Scenera. Good luck!