This is insane and totally messed up. You broke up with your boyfriend, great first step. Tell your parents ASAP, tell them exactly what you told us. Cut ties with your Nan, she is not a good person. By cutting ties, don’t go and see her, don’t call/answer her phone call, avoid seeing her in person. There is no excuse for there behaviour. I am so sorry this happened.
What the heck was there reaction when you caught them? Have either tried to contact you since you left? How long are you able to stay with your friend? Will your parents let you stay with them?
Normally I would think this was fake but I actually believe this one.
I’m so sorry it happened.
Ditch your boyfriend, tell your parents what happened, and cut your nan off.
Your boyfriend fucked your grandmother?? My mind is so messed up right now. This is insane