How to get rid of the friend’s face

Faces never stay if you lose a level, but there are ways to collect faces faster:

• The Show a Friend level is short and easy, and if you don't mind cheating you can play it without a friend.
• Once you play Show a Friend three times, the Bonus Stage is unlocked. In this stage faces fall forever and you only need to juggle and destroy one to have it considered "won".
• Leaving the Surprise Snaps mode on will have the camera take pictures of faces it detects while you're playing the game. This will let you get more than one new face per round.
• Finally, sometimes a UFO "captures" faces it detects in the regular 3DS camera album. You still need to play a level with them to unlock them completely, but you can take pictures when you don't have time to play and then clear the levels when you do.

Yep, you need to go to the 3DS camera and merge the faces. That's the requirement for unlocking the Bonus Level.

The steps for doing so are as follows:

1. Go to the 3DS Camera.
2. Flip the switch on the bottom right corner of the screen.
3. On the menu that appears, tap the camera icon (located on the rightmost side).
4. On the next menu, select the head with a question mark icon.
5. Find someone (or something) and take a picture of it.

Do it a few times and it should unlock the bonus stage.

If it doesn't, try the following: Look to see if the UFO brings some merged faces over to the main stages. If so, play that stage, and locate and destroy the UFO with the merged face. This will "rescue" that face. After successfully completing the stage with the rescuing, you should unlock the Bonus Level (provided you completed all previous stages).

Hope this answer helps!