I used to live across the street from some people who had a son my middle daughter's age. They were never really good friends, but when they were younger they played together a little.
When I gave my children a birthday party I usually invited the neighbors as well as their friends from school, but the year my daughter turned thirteen, she said she would prefer to just have some school friends and two or three of her friends on the block.
Not wanting to hurt my neighbors feelings, I never mentioned the party to her...but the day of the party I was over at my other neighbors picking up some coupons for the pizza they were going to have, and she happened to stop by.
She says...."Oh, you are getting pizza tonight?" and I said yes. I didn’t want to tell her about the party....but she probably already knew about it....because she kept persisting...."Oh..just you and L are going to have a pizza all to yourselves?" Not wanting to out and out lie to her, I said, "Well actually, its her birthday and she is having a couple of friends over for pizza".
My neighbor said, "That’s nice" and that was the end of it.
Later on that evening, during the party, I come downstairs, and who is standing there, in my house, eating a piece of pizza and drinking a soda, but her son....the one who wasn’t invited!
My daughter comes down behind me, and sees him....and she said...."What are
YOU doing here?" He said that his mother sent him over.
I didn’t want her to hurt his feelings so....I shushed my daughter, and I tried to make him feel welcome......he was just a kid, after all......but I was pretty offended. I mean, he just walked right in and crashed my daughters party!
The worst thing.....he didn’t even bring my daughter a gift and he ate twice as much as the other kids!
Kids0527-031. Teenaged boys eat three times more than elephants so he's not unusual. 
2. The poor kid was simply doing what his mother told him to do and landed right in the middle of a major faux pas that was not entirely of his own making. I hope you took the opportunity to instruct your daughter in how to act gracious in similar future circumstances.Asharah's comment: I hope OP had a little talk with neighbor mom the next day. Were there other boys there or was it an all-girl event? Can you imagine if it was a slumber party he crashed?