Hi Willow (haven't forgotten the harold, sorry!
just been unwell, so not got out)..
I have had a similar situtation in relation to sign, (the ASD specialist school are very hot on PECs, and said they use it with all 'their' kids. but when I could demonstrate that my ds already had the level of communicaiton they were ultimately aiming for (albeit signed) they had to back off, the idea that 'we use PECs' because the setting offers snacks with a symbol card is very prevelant. [cross].
Rather than providing evidence to them it might be worth asking them to provide the evidence base for this working with kids with language abilties like your ds, (chances are they won't have it).
Just a thought, It might be worth going on the PECs course... then you would have evidence of what is was supposed to be, so you would be able to express an opinion on what they are doing. They should also be able to give you overview of why/when it does work/help.
PS I am not anti PECs, IYSWIM, I know it is great for some, but one size fits all it is not..