Telling your friend the truth risks the friendship. Not telling them risks the friend.

Both options carry a risk, and the two risks are not the same. Staying quiet protects the relationship for now and leaves your friend alone with something they may have wanted to know. Saying it puts the friendship under strain for a while, which is a real cost, and it is the only version where they get the information.

Say it once, privately, with no audience and no repeating. Make clear you are on their side, then let them do whatever they choose with it. What you owe is the honest sentence, not the outcome.