Although the parents will obviously be trying to do their best for their child, sometimes we (all) need to hear advice and opinions from lots of people to help us make the right choices.
It's hard to know what is going to be the best school to choose for any child (as you can tell from the daily threads on MN about it) , but much more so when your child has additional needs.
What I see happen a lot, is parents looking at one place, deciding they like / don't like it (often for the oddest of reasons, sometimes for very sensible reasons) and making a decision based on that. Unfortunately, there is no service in our LA that has the capacity to take the parents round to several different options, and help them ask the right questions, and to point out things that are going to be relevant to their particular child this year, next year, and in 5 years time. Parents have to make decisions without full information. They will also often have one person (who themselves with have an axe to grind, or indeed, they only have partial knowledge) who is bending their ear, or, sometimes who they have great respect for, and they will listen to them above all others.
Still, in order to move to a special school place in our authority, the provision panel have to approve it, you can't just apply direct to a school because thy have a place to fill. If it's the same where you are, then presumably the panel thought it was a good placement.
It is a minefield though. I think I might just chat at the end of the day if it were me - "I hear he's going to be leaving us soon ?" and see what the Mum says without making it sound like you are criticising. Probably not what others would advise, but I couldn't just watch and say nothing, personally. I get too attached to the pupils, and want the "best" for them, just as the parents do. Sometimes though, you just have to accept that your idea of what's best and the parents' idea of what's best, are just not going to marry up, but at least you've given them things to chew over, and can be contenet you did what you could, while they were with you.