@Roberta,
Roberta wrote:
My first thought was that he was invited and was being treated by his friends. If that's the case, it's not up to him to invite you.
Hmm, your and eBeth's replies surprised me, on this particular point.
If I organized, or helped organize, something for a friend's birthday, and that friend has a girlfriend/boyfriend that I knew about...
1) It would go without saying that they're welcome as well;
2) If I was unthinking enough not to explicitly invite them myself, it'd seem totally natural/OK for my friend to say, "hey my boy/girlfriend's coming too, that OK?" Of course that's OK.
The only exceptions for not inviting their boy/girlfriend, or not being OK with my friend taking their boy/girlfriend along, that I can think of are:
a) If I knew the boy/girlfriend, and they've proven to be a real pain and we had some bad experiences hanging out together. (But in that case I would leave them together on my friend's birthday itself, and just invite my friend to celebrate on another day);
b) If my friend actually told me s/he was looking to do something without their boy/girlfriend;
c) If it's not a "let's go out together & have fun" kind of thing but more of a "let's hang out & talk about personal stuff" occasion. But that wouldn't likely be on their birthday..