[Q] Warranty and Bootloader topic

My friend has an Xperia S... and he wants a custom rom in it ( he wants a Kit-kat rom ) and i will be the one to put it ( he doenst have knowledge about android )

so this is i gonna do ( if possible )


1. back up DRM keys
2. unlock bootloader
3. install recovery
4. install rom and gapps ( pureslim rom )
5. relock bootloader ( usng DRM keys ) after successful flash


Questions:

1. can he retain the warranty if ever i restore the DRM keys?

2. is my method correct, or im missing something?...

:)