In this paper, we present an implementation of the FairPlay framework for secure two-party function computation on Android smartphones, which we call MobileFairPlay. MobileFairPlay allows high-level programming of several secure two-party protocols, including protocols for the Millionaire problem, set intersection, computation of Jaccard similarity coefficient, etc. All these functions are useful in the context of mobile social networks and opportunistic networks, where parties are often requested to exchange sensitive information (list of contacts, interest profiles, etc.) to optimize network operation. To demonstrate the feasibility of MobileFairPlay, we present an application to privacy-preserving interest-casting in opportunistic networks, implementing a recently proposed protocol. We tested running times of the implemented protocol on several Android phones, obtaining very reasonable (up to $5sec$) running times. These results clearly promote MobileFairPlay as a feasible security framework for mobile environments.