In a certain code language, 'my best friend' is written as 'ck rl mk' and 'friend is life' is written as 'rt mp ck'. How is 'friend' written in the given language?

Let's write down the sentences and their respective codes:
'my best friend' → 'ck rl mk'

'friend is life' → 'rt mp ck'

Comparing both statements, the common word is friend.
Looking at the code sets, the only common code between 'ck rl mk' and 'rt mp ck' is ck.

Therefore, 'friend' is coded as ck.