Permission to attach to a neighbours wall

No they can’t do that legally there should be some sort of agreement wether legal or “man to man” agreement but they probably assumed you would be okay with it as they used to be connected and I’d go one further then that and say you can bet your lucky socks the contractor didn’t ask your neighbour if he can do this either and just assumed off there own backs so may not entirely be your neighbours fault. However I’m no expert in wall construction or retaining walls but I think it makes no difference connected or separated nor do I think you can blame neither one or the other for the wall collapsing unless one of you let your wall degrade beyond repair etc blown bricks and lost and missing pointing

No they can't just attach to your wall without permission, (party wall agreement ) Although joining the two wall would probably make them stronger

Your neighbour should of contacted you first as he had no right to connect his wall to your wall if he was concerned about the strength of his wall where it meets yours he should of increased the size of his wall at that point (eg extra pillar's )

I am assuming that the original wall did both properties. If you have rebuilt your wall with a stop end ie a vertical end to your wall then if the neighbour does the same you will be fine. But if they have" tied in" to your wall there is the potential that if there is movement again from neighbours side it might effect yours. But to stress if both walls have stopped ends even if mortar is attached to yours it will be fine as long as no "tie in" has accured.