It's a fact of life with parties that some parents will stay and others wont. I don't mind parents staying, in fact I prefer it if they don't - unless I know them - as otherwise it always feels a bit awkward and uncomfortable. However I've always made sure I have enough people there to help out, and to ensure if there is a child who gets upset, falls out of the tree (this happened at DS2's last party....it was a small tree honest ) then either I can sit with them until they're feeling better & able to join in again or else one of my 'helpers' can. I've always been lucky in that they've never stayed sad for long.
Without having spoken to the little girl's dad, you don't know the reasons he dropped her off and left.
He may have had something urgent/essential to do and/or
He may have genuinely thought she would have calmed down after a few mins
He would probably feel terrible if he knew his DD had been upset - I would mention it but not in a judgey way, just so he knows that at the next party he might want to a) stick around for a while or b) if he does have to go, leave a contact number so if she is too distressed, he can be phoned........