What would you do if your best friend's live-in partner found out that you knew that she's (ur bff) cheating?

You've been best of friends for 12 years and she's been living with her live-in partner for 7 years. Your best friends live-on partner suddenly change and always with his friends gambling and partying. When your bestfriend talked to him about it he got angry and hurt her. Your best friend love this guy so much and she can't leave him but she feel so lonely that when she'd met another guy she had a secret relationship with him. She dont love this new guy and still love her live-in partner. Your best friend told you this situation and give her a friendly advice but didnt stop her. One day you receive a text msg you thought its your bestfriend, the sender ask you what do I think with d new guy and you replied of what you think.. then your bestfriend called asking you not to answer any call or text msg bec her phone is not with her the live-in partner is using it. But its too late. If you are the best friend, what would you? Would you still deny it to her live-in partner that you knew she is cheating even he knew already?