Use 'unchangeable' to describe something that will remain the same forever and cannot be modified. He viewed their friendship as unchangeable, no matter what happened.
Pro Tip 2/3
Certainty and Emphasis
To express certainty or emphasize that something cannot be questioned or reversed, use 'unchangeable'. The verdict was made; the judge's decision was unchangeable.
Pro Tip 3/3
Seriousness Context
Often used in serious or formal contexts such as discussions about principles, rules, or natural laws. The terms of the contract were unchangeable once signed.