my mom (50) and friend

i was living with my single mom (50) and i had a close friend who was visiting me often. By the time i realized he was always finding a way to be alone with my mom for a moment so i wanted to check whats going on and i heard them making out in the kitchen. So i was just found out he was fucking my mom and i dont even know since when and how. I tried to peek on it and saw them kissing while he was grabbing her ass. i was used to my mom does sex often in her relationships but never guessed this would happen. I took some more time out of kitchen and watched it. They agreed to do something in the night when i fall asleep. And in the night time i saw him walking to her room and when i listened to her room i heard my moms moans under him. They tried many positions and He finished with cuming in her mouth.I somehow enjoyed this situation so invited him even more after that day