I would say something like "I've said no and I mean no and I am not arguing with you anymore".
And ignore her completely if she mentions it again.
I think sometimes, we overthink and overtalk things when really kids just need to be told (nicely) they're being ridiculous and to STFU.
Agree with a PP = I'd be horrified and embarrassed if someone spent $90 on my child, apart from family.
There is no way I could afford to reciprocate that kind of gift and then I'd feel awful!
Agree with both of these. $90 is more than I'd spend on a single present for my own kids, let alone someone else's. Wow! Lucky friend indeed.
This is a non issue though. There is one word - "No." She's not having it. She's not getting her own.
Just a side thought though - is she likely to spoil the party by having a huge tantrum when this friend opens the present? Or to maliciously damage it on the day?