DS is in mainstream as part of a full time ABA programme. In effect he has a specialist education but part of it takes place in mainstream (and part in 1:1 at home / at the ABA provider office). It works well and is better option for him than SS as there are no asd SS or units here (he was refused place at SS nursery at 2 as deemed too bright but really it was just to gatekeep places, they offered SS a few years later by which time it wasn't suitable as he had made too much progress and would not have had peer group of same ability there)
I would say that yes it is bullying because it's not the full story. You have a legal right to mainstream instead of SS and the LA must put in the support necessary to make mainstream work. The difficulty is unless you have tried and failed in mainstream with LA services and / or SS few LAs will jump to mainstream with ABA without a battle / tribunal.
So in reality the choice initially is SS nursery or 15 hours mainstream nursery with perhaps 10 hours 1:1. You will most likely have to appeal / go to tribunal for more.
ABA yahoo site is good to ask for experiences of your LA. Some will negotiate on ABA and some won't. Your friend will have to argue ABA is the necessary step to exercising right to mainstream. It can be a long and expensive battle. Under the sen changes she can ask for personal budget so take the cost of SS and ask to use it for ABA in mainstream. But LAs don't have to agree.
DS ABA provider is very pro mainstream and DS is academically able enough to be there. He does have all the language problems other people describe here but that is addressed by skilled ABA staff breaking down and differentiating the work for him. He has almost caught up in maths, his reading and spelling is age appropriate and his writing is behind but not dramatically so, he's not the least able child in the class. Even in mainstream the ability range these days is vast. For social skills etc the pool of children in mainstream is great, he has limited social interest (but that's the same with the asd kids he sees at ABA provider), but the children are good with him and persistent. He has learnt to copy them and follows what they do when he's unsure.
DS could not be in mainstream without ABA, because mainstream teachers and TAs do not have enough time or training to know how to teach him. They rely hugely on ABA supervisor to do lots of it (social language, behaviour, life skills) and they just provide teaching for literacy, maths etc (even then they need help in adjusting it). Mainstream without ABA would be miserable, but with ABA he is doing well. He has built up time slowly and still doesn't go full time, this avoids it being overwhelming. He also does some 1:1 pull outs during school day but only 15 mins a few times a week now, mostly when he's in school he's in the class. But the great thing about having ABA staff in school is it's completely individualised to him.
If your friend gets full time 1:1 in mainstream (ABA or not ABA) that is more expensive than SS place usually (unless big transport costs to SS). So if later wanted to switch to SS then there is a good chance that would be possible. Not all SS nurseries are full, here one SS didn't have a child under 7 in it when we looked round (it can take them from 2) because of the gatekeeping.
Far more children move from mainstream to SS than the other way. Here they pour in at 8+ often having made zero progress in mainstream with poor provision or unmanageable behaviour. She may not be able to get a place within a few weeks or months if she changes her mind but usually you can in the medium term. It is variable by area. If it's cheaper than the alternative places have a habit of becoming available. It sounds like they are using it to put pressure on her. Probably because they know she is doing ABA and don't want the expense of that.
Many LAs will refuse full time 1:1 in mainstream, you just have to appeal. Here the only children who get proper provision are those whose parents appeal. Our LA offered 75% 1:1 in mainstream but backed down and gave 100% as soon as we lodged the appeal. then we had to appeal again to get ABA on the statement. For that we had to go to tribunal.
If she trusts her ABA provider and they are advising mainstream then she should give it a go, but expect to have to fight for it. Some ABA providers work a lot in mainstream and make it a success.
Our ABA provider I suppose had a vested interest but it's actually part of their philosophy for every child to be in mainstream if possible so I didn't feel like the advice was due to any self interest on their part. Other non ABA approaches eg SCERTS also recommend children go to mainstream / mix with typically dev children as part of their philosophy. Our ABA provider also works in some SS but it's harder to get ABA in there than mainstream.
Sorry that's so long. Just wanted to say with the right support it can work in mainstream - but getting the right support is very hard.