your friends will forgive you quicker than she will
If you really care about your current relationship, you should not disregard your girlfriend for a friend she doesn't "get along well with". It's sad but sometimes relationships necessarily shadow friendships. Your friend overseas should understand your situation.
why do you care what they do when you already have plans?
Your expectation of your girlfriend is correct...even if she doesn't get along with these people she has to understand they're your friends and like someone else said: Weddings happen only once (hopefully).<br />
You guys could go to this wedding and then go out and celebrate your girlfriend's birthday. <br />
If she doesn't understand and is so damn selfish maybe you should consider finding someone that will want to make YOU happy.
Make it up to her - take her out for dinner or a shopping trip. Seeing someone ON their birthday isn't that big of a deal. The main thing is that you make an effort to spoil her some time around her birthday, doesn't have to be ON the day.<br />
A wedding DAY isn't something you can make up for later. It's one day. and once in a lifetime (hopefully!)<br />
(Sorry, I only just realised it was you GF's b'day, not your friend - <br />
I understand her hurt, but surely she can understand and see you later that night or something...?<br />
Just be gentle with her and explain how much it means to you that you be at your mates wedding, but also emphasis how much SHE and her special day means to you too and that you want to do something really special with her later on.)