To Tell or Not To Tell

Let me start by saying I am very close with my mother so needless to say, when my husband and I got together, I couldn't keep away from his mother.. She is a nonjudgmental, understanding, and supportive sounding board that I am so very Thankful for. Needless to say, I found out that she is sick because my father in-law put me as her second emergency contact.. They couldn't get ahold of him so the hospital called me (we didn't even know she was in the hospital). When I asked them what was going on, what her diagnosis was, they told me that her COPD was acting up and that she was having shortness of breath, chest pains, and a very fast heart rate and she was diagnosed with Afib. They were putting her on a new med and needed to make us aware. I called the hospital to talk to her and she was open and honest about it.. Why.. Because she got "caught" when the hospital called me.. Lol.. Almost like a child getting caught. She asked me not to say anything to my husband nor her daughter. I told her I wouldn't say anything to my SIL but I didn't feel comfortable keeping it from my husband. She has to be on oxygen now because of how bad her COPD has gotten and she is having more bad days than good days. She said she wants to tell him and that she will tell him.. She just wanted to do it in person when we go to visit her at the end of the month. I told her to let me think about it and I will let her know tomorrow.

What should I do. Should I tell my husband? I really want to and feel like I should but she is such a private person and was in tears.. Idk what to do. We have been together for 13yrs, been married 11. Thank You for your answers.