When the user chooses to install Fiddler2's certificate as a trusted root certificate, he is then choosing to compromise his own security. I'm not sure there's much you can do about it, since your application's HTTPS connexion will go through Android's certificate validation system, which will consider the connexion as valid, since the certificated is trusted.
The solution I would go for is embedding your SSL certificate in your application, and tell your application it is the only trusted certificate. It's secure and free, as you can attach a self-signed certificate you created yourself, since you control the verification mechanism. See this blog article for code example.