I ruined a surprise farewell party. How do I make it up for them?

My friends threw this farewell party since my friend is leaving soon. They sent us an invitation message like 2 weeks ago and I completely forgot it was a surpise so I mentioned it in front of the person who wasn't supposed to know. Now his girlfriend is really mad at me, the party is tomorow at her house. I apologized like 10 times and told them I completely forgot. How do I make it up for them? What should get with me tomorow? I need to buy her something at least!!