I can understand a child being fairly confident when relaxed and comfortable, playing with children they know very well, but the exact opposite in a group where they would only know the host.
I'm still like that now. I'd rather have a tooth out than walk into a room where I don't know anyone. Good job I don't have the option, or I'd be toothless by now! 
Maybe the mum has had this kind of situation before with her dc? I know from experience it's not very relaxing for anyone, if there is an unhappy or clingy dc at a party, who doesn't feel able to join in.
The fact that the invitation was accepted at first, probably indicates your friend's initial intention for her dc to come to the party. I imagine there's been some subsequent discussion with dc in which they have said they don't want to go. What's a parent to do? Have also found myself in that situation in the past!
I wouldn't see it as a slight. It's just that children start having their own opinions about where they go, and who with, and parents are wise to try and rise above it. Your friend's dc probably still want to be friends with your dc, just doesn't want to be the 'odd one out' in this situation.
If your dc does get a reciprocal invitation, don't turn it down 'tit for tat'.
Your dc might not actually mind not knowing anyone. Apparently that can happen. 