Extremely rude, nigh on unforgivable from someone whom you consider a friend.
I can remember some years ago receiving an invite from a friend to 'come over, to join them and friends for a small party tol finish up the Christmas food. We assumed drinks and nibbles so had supper as usual. We got there to find it was a three course meal - and we ate it, no apologies, probably ate a bit less than usual, but I would never ever have admitted to eating before we came, even, as in our case, inadvertently. The height of bad manners.