I don't know what the etiquette is, but I would buy a gift. Since you've gotten closer, just be honest that you got caught up in other things and forgot. Have the gift with you at your party and give it to her there.
A true friend will understand.
Also, a lot of people feel that just becuase you can't attend a party means you don't have to buy a gift...I don't agree. I'm sure I'm in the minority, but we always give a little something, even a $3 or $4 gift from Walgreen's...just a little something to acknowledge the special person.
Best wishes,
J.