My girlfriend of ten months kiss another guy what should I do? We are engaged.?

My girlfriend Sabrina went to Portugal for a two week vacation to see her family. I couldnt go because of work. While she was at a festival in the town shes in. She kissed another guy. She said she didnt like him even called him ugly. But me and the guy are friends on FaceBook and he told me he kissed her. He put the blame on her and said she wanted to kiss him. I really love her but I am so confused.