They are always honest with each other because they don't like to ----- mutual trust.
Ruin, endanger, risk, and imperil are a couple of suggestions that I have seen used on the Internet. But I am looking for the most idiomatic verb for sentences such as the above.
Undermine is suitable in this case.
Mutual trust is wording that is used to describe established relationships where trust has built up over a period of time.
Therefore, anything which poses a risk to that trust is something that undermines the foundations of that firmly established relationship.
The word undermine is used on Wikipedia in relation to the legal term 'Mutual Trust and Confidence' expressed in UK law.
"Betray" is probably the best general verb.
Its main usage is to mean to break faith or agreement.
Perfidy
2. An act or instance of disloyalty
https://www.merriam-webster.com/dictionary/perfidy
Verbify: commit perfidy or commit a perfidious act.
