Father cheating on sick mom

Today I stumbled upon something that is tearing me apart. I opened up a family laptop and saw that my stepfather of 6 years has been emailing various women and was getting emails from a sugar babies website. I looked up one of the emails where had a conversation with the person about wanting to “meet up” and schedule an appointment. The email led back to an escort website. I then read another email where he asked another lady if she remembered him from last Tuesday. The answer seems obvious .... tell my mom right away right?! I’m 19 years old and have very little attachment to my stepfather so should be easy right?! Wrong. In the past few months my mom has been struggling with breast cancer, last year she had a tumor and the 5 years before that she had cancer. she’s going through a lot. Which infuriated me so much more that a man could do such a thing to such a strong women who has been through hell and back. Today my mom is supposed to be finding out if she needs chemotherapy or not and now I have to also somehow tell her her husband is sleeping with other women or at least talking to other women considering sleeping with them.... I really need advice and help. There’s no question about whether or not I tell her .. the question is when is the right time.... and how