Mustard is a simple and easy to use Twitter and status.net client for Android phones. As almost all status.net clients, it can also be used with Friendica, the great decentralized social network. This very short tutorial will show you how to set it up:
Add your Friendica password (2.)
Add the URL of the Friendica server you are using (3.)
Choose Force use of TLS/SSL for better security (4.)
Click on Login (5.)
Mustard is a good way to start, but if you want to use a more elaborated client, I recommend to use Twidere instead. You can learn how to use Twidere with Friendica here.