What is Japanese version of the pun word, "slife"? ... and term for adult friend?
It's a play on 人生 (jinsei, lit. human life) with スライム生 (suraimusei, lit. slime life).

'Slife' is an accurate portmanteau here. Since it's Slime + Life.

Friends are friends regardless of age. There are more formal words like 友人 (yujin), but how it's used is circumstantial and highly contextual in nature.