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I'm not sure that you can. There are Bluetooth keyboards for Windows 8 and RT devices. http://www.logitech.com/en-us/tablet-accessories/keyboards/tablet-keyboa...
I don't know why Windows 7.5 would not have the capability built in, but as far as I know it's not possible. I know that it didn't used to be; I found an article on Windows Phone 7 (which isn't 7.5 obviously).
How about a USB keyboard? You would need to find one with very low power consumption though.
Oh, and it Nokia Lumia, not Lumina. The Lumina was a crappy car made by Chevy in the 1990s. The Nokia Lumia is much better looking. ;-)
My understanding from this Windows Phone Central article is that Bluetooth functionality for Windows Phones up to version 7.5 was limited to hands-free headsets and speakers. Windows 8 is supposed to have expanded Bluetooth functionality, which unfortunately doesn't do your friend with the Lumia 900 any good.