You can't because the link is permanent. This is noted when you link your device for the first time:
You can however start a new Clash of Clans village by following the instructions in this answer.
I also recommend you read this post on the forums as it explains really well how the device linking works.
Yes this is possible. What your friend is going to need are at least two Google accounts linked to his phone.
First, from within Clash of Clans, go into settings and connect the current village to a Google+ account. This will allow him to restore his village from any device that he signs into his Google account from.
Then in Android,go to settings -> Apps -> Clash of Clans, and 'Clear Data'. This will wipe the village off the device, but it will still be linked to the Google+ account.
Now open Clash of Clans again and get past the intro tutorial. On your iOS device, open Clash of Clans, go to Settings -> Link a Device -> This is the old device. On the Android phone, open Clash of Clans, go to settings -> Link a Device -> This is the new device. Follow the steps to link the Android phone to your village. Now connect the new village to a second Google+ account (Must be different than the first one!)
Now any time he wants to switch between the two, he just simply needs to Clear Data as explained above. After loading Clash of Clans, he will be able to sign into a Google+ account (there's now an option to do so without doing the tutorial), and restore its respective village.
I just tried this entire procedure and verified it works.
For future reference, it appears that Android phones will soon be getting user profiles. So that like Android tablets, different user profiles store different sets of user data. When this feature comes out, it would be possible to setup each village on a different user profile and just simply switch between the profiles, no clearing user data needed. Though I would always recommend linking each village to a Google+ account for safety.
Best Answer
First and foremost, you won't be able to use the Google+ account linking on the iPad. However there is still a way to recover it. Prior to doing so, I would recommend first creating your own Game Center account on the iPad. This way both you and your brother can play on your own separate villages.
To get access to the account again, you may need access to an Android device. I would suggest borrowing a friend's phone for this. If the device in question is already playing Clash of Clans, you will want to ensure to link it to a different Google+ account prior to recovering your own; this will ensure that it can be recovered once you are done.
After that, open up the System Settings then open up Apps. Locate the Clash of Clans app, and Clear Data. This will remove the existing village from the device.
Then after opening Clash of Clans, you should see a button to sign into Google+. Sign into your Google+ account to recover your village.
Now you can go into the Clash of Clans settings and choose "Link a device". Do the same on the iPad (using your own Game Center account), and follow the instructions to link your village to the iPad. If done successfully, your village should now be on the iPad and linked to your Game Center account.
Finally, you can go about restoring the existing village on the other Android device. By clearing the data again, you can then sign into the existing player's Google+ account to restore their village. Of course if they didn't play Clash of Clans originally, you can just simply clear the data to remove their access to your village.