My cousin brother molested my childhood best friend

i am 19f now and i need to get this off my chest when we were kids around 9 years old so about 10 years ago my cousin brother molested my best friend who i'll call Lucky (fake names) we were just children then too young to fully understand what was happening or how to speak up about it, It happened when my cousin brother who I'll call J (fake alphabet) lured my best friend into our parent's house by giving an excuse to play gta vc because only few of us had computers that ran gta games that time so he (cousin brother) lured him (my best friend) into the house and then he pushed him against the wall facing the wall and he tried to sexually harrass him i just came in a second inside the house and lucky ran away from the house crying, and J was panicking at that that time, i was freezed there, then after a second my cousin brother told me to go outside for a while and come back after an hour i went outside and started walking in colony and came home after an hour my cousin brother went back to his home and my best friend's parents told my parents that this was such unexpected behaviour from a best friend and they left the city next day, at 5 am i didn't got chance to say sorry to him or atleast to talk to him, i didn't realised what has happened but now after years of my best friend left me i feel so bad so guilty so angry against my cousin brother and when i told my parents my parents did nothing but told me to beware of my cousin brother that's it. Fahh! it's something that's stayed with me even though it happened to him and not to me i keep thinking about how it must have affected him and how none of us had the words back then to name what was wrong.