So sorry. That sucks.
My SIL did that to me with my older DS when it was my nephew's bday. Happy birthday, cake and his presents. Um, it was like the whole party happened when I stepped out for 30 min. The odd thing was that there were only like 6 adults and 2 kids there, so it wasn't like if they waited for me they were holding up a bunch of people. I feel you.