Katy Perry's best friend Rihanna pulled out all the stops for her pal's bachelorette party in Las Vegas over the weekend. First, friends joined the pop star, who is engaged to Russell Brand, at the Hard Rock Hotel Beach Club's pool. Perry wore a white bikini (she is the bride-to-be after all) and Rihanna chose a sexy metallic maillot. For the evening's celebrations, the group headed to the MGM Grand, where they ate at Ago and checked out a Cirque du Soleil show. This time Rihanna opted for the all-white ensemble, while Perry chose a sparking black minidress and festive veil. The party went late into the night, ending with a cake delivery for Perry in the group's Casino Tower suite. Congrats, Katy!