I feel like she betrayed me though I am not sure?

My fiancee and I are going to graduate college in May. We plan on getting married in the fall. She was asked by my former roommate to pose for his fraternity publication. She knows I don't like him and we don't speak. She wasn't sure and asked what I thought. I said she should not do it and I was mad that he asked her. I now have found out that she did do it despite what I said. I feel like she didn't listen to me and sided with him and then helped him get what he wanted. I'm also concerned about the fallout of her choice. Am I right or am I unreasonable?