When considering doing something you think should be kept secret from someone whose trust you value there are two basic considerations:
1) Apply the Golden Rule. How would you feel if the you discovered the secret someone you trusted was keeping from you.
2) Secrets are inherently dangerous. If disclosure of something we want to keep secret from someone we have a trust relationship with will probably upset, disappoint or outrage the other person or create a sense of betrayal, it better be more important than the relationship because its disclosure is at least possible and, in this day and age, probably likely, and damage to the relationship is inevitable. It is reckless to let a valued relationship hang on the slim thread of hope that the secret will survive indefinitely. Thus, be very sure that the jeopardy you are putting your relationship in is worth it.