Caught mom with guy in shower

I (18m), my mom (56f) and my dad (62m) are renting out a room in our house to a Nigerian foreign student (22m). He’s very nice and friendly and has always been kind to me.

Yesterday I came home after school and went up to my room. Going up the stairs, I heard moaning and laughing. As I passed the main bathroom, the door was slightly open.

I saw my mom and the student having sex in the shower, with lots of passionate making out. I froze for a moment but left the house then and didn’t come home for a few hours.

They definitely don’t know I saw them. But I’m still not sure what to do.