My friend asked me to lie for her affair

Dear Delyara, My closest friend is having an affair and has begun using my name as cover. She tells her husband she is with me, then sends a message saying, “If he asks, we had dinner.” At first, I agreed because she said her marriage was already over and she needed time to plan. Six months later, nothing has changed. Her husband thanked me last week for supporting her through a difficult period. He looked exhausted and sincere. I felt sick. I have told my friend I will not lie again. She accused me of judging her and said friendship means protecting each other during messy times. We have known one another since university. She held my hand during my miscarriage and slept on my sofa when my father died. Now every good thing she has ever done feels tangled with what she is asking me to become. If I tell her husband, I betray her. If I remain silent, I continue helping her betray him. I did not begin the affair, but my name is now inside it.