I see my mom do this

I’m 25 years old and live alone with my mother after my father passed away. Recently, she’s become withdrawn and prefers to stay in her room alone. For some reason,

I picked up her phone and found she was chatting with a young man who, from his picture, seems to be my age. They were exchanging a lot of romantic messages,

which really bothered me. So, after she fell asleep, I took her phone and discovered a huge amount of text between them,

full of romantic and sexual content. The relationship had been going on for eight months. The shock came when I found they were exchanging nude photos.

I was in a state of shock that lasted for days. What can I do? Should I tell one of my relatives or talk to her?