When DS was in Y2 he was told by one of his friends that he was going to be invited to his party, he was looking forward to it. Party came and went, no invite. I must have asked him when the party was as I realised he hadn't mentioned it - he just said that his friend had realised that he had got carried away and invited all the people he wanted to invite without realising that he had a limited number of spaces so several people couldn't go.
He was fine about it - and I wouldn't have dreamt of saying to the mum - what's up, he was invited, now he's not... because he never got a proper invite, it was just playground talk.
And reading this - sounds like your dd who did have a proper invite, is being pushed out for someone who just had a playground invite.
Given that there is the complication of 2 parents doing the inviting and one seems to be under the impression that your dd is going and is pleased about that, it definitely seems reasonable to talk to her to find out what is going on. Might be that there were 3 people that they jointly wanted to invite plus one extra girl each and the other girl is trying to manipulate it, you just don't know.
I would be tempted to say to dd in the morning that there seems to be some confusion as nice mum thinks she is going and is pleased she can come whereas nasty mum 2 thinks that she shouldn't have been invited. As it could get unpleasant in the playground if nice mum has said that your dd will be there and mum 2 has told her dd that you are not coming - at least that way your dd will be prepared and be able to say something about it being not clear or not sorted yet, rather than being forced into saying that she won't go (which for all we know the dd of the nice mum might be very upset by but the other girl would then take your dd's saying she's not going as extra validation for her other friend going). It does sound a shame if she does have to miss out as it sounds like it would be a fun thing for her to do, particularly with her group of friends - so even if she were to go at a later date, it wouldn't be the same as going with them.