Was it wrong to dress better than my male friend's girlfriend at a party if I'm single?

I wasn't dressed provocatively. It was his birthday. I felt I was dressed more appropriately in a stylish outfit for the occasion. There were only a few other women who were also dressy. Unfortunately, his girlfriend was giving me evil looks all evening. She probably gave the others the same as well. He tried to reassure her he only had eyes for her, but I really felt guilty! Should I downplay myself next time so she stands out as his girlfriend at events? There were other eligible men at the party I thought I might meet. I wanted to look presentable and attractive. I wanted to look nice for my friend too. Was this wrong to do on his birthday? Anyone ever had this experience?