Beautiful drawing! I made a costume sketch inspired by her recently myself. It's not nearly as advanced as this, but I still wish I could show it to her. I'm sure your friend adores it, and Nicki Minaj would be flattered herself.
Thankyou, and to answer your question, I don't remember exactly. I referenced 2-3 photographs, but I can't find the source. I did take a screenshot with the references in them however, which you can find here : Link The costume and colour scheme was referenced by Nicki Minaj's Pink Friday photo shoot. Good luck!