Very easy. All your friend has to do is to stand inside his Linden home. From the world menu, choose "abandon land" (make sure he takes is own stuff back to his inventory before). Then he will be able to choose another house as he/she did the first time from the 4 different available themes.
You can change linden home up to 5 times a day. Do not overuse that or your friend whould be blocked for a few days to choose another home.
Abandoning or changing your Linden Home
To change your choice of Linden Home type, you must abandon your current Linden Home and then register for a new one.
To abandon your Linden Home:
Choose World > About Land and click the General tab.
Click Abandon Land.
After you confirm your choice, the land is removed from your account.
Now if you wish, you can register for another Linden Home.
Warning: Please use this reasonably: if you abandon and choose a new Linden Home five times in 24 hours, you are temporarily blocked from choosing another. You will need to wait 24 hours before registering again.