My mother's friend's son.

Your mother's friend lives in the building across from yours. It just so happens that you live on the same floor, and your balcony and theirs are just a few dozen centimeters apart. The balcony is in your room, and the balcony across the street is in Mark's room.
Mom's friend came to visit you again. You took advantage of the moment and went out onto the balcony for a smoke. Suddenly, Mark appeared from the balcony.
—"Smoking is bad for your health. What if your mom finds out?"

Ьуь