Advice please?friends problem?

It's my frends birthday tomorrow and he's hvin a party but he's not allowed invite aload of people so he left one of us out and she woz rely upset about it.he said it's coz his other frends don't like her or want her der and woz really nasty about it which isn't like him.when she said I thought we were frends he woz like Wel u thought wrong. I know he's doin it bcos of his frends but it made me rely angry dat he'd be like dat and exclude her hu he's been frends wit for years.do I go to his party or say I'm not going bcos he's leaving her out and it's not fair.I don't want to miss his birthday but I don't like the way he treated my frend who has always been der for him