I'm disappointed/pissed that my best friend started watching my favorite show with her boyfriend
How long have you been friends with them?
I've been friends with them for a couple of years.
Your friend can watch whatever show she wants to with who she wants to. You cannot control her.
But I don't want to watch it with that guy, I can't understand everything he says and I don't want to watch it without her.
Find a time when she is not with her boyfriend, then you can watch it with her. Expecting her to wait for you all the time is unfair to her.