You just don't. If consider him your best friend you definitly wouldn't want to do that to him, and just to be straight-forward, she's with who she's with for a reason. So why hurt your friend relationship for someone who's not promised to be there like your best friend is?
maybe she is jealous
There really isn't a Spanish word for "besties", but you can say "Mejores amigas" (Best girl friends) or "Mejores amigos" (Best guy friends or best guy/girl friends).
Yes, it would be. She would probably not stay your best friend for very long if you did that.
either bff for best friends forever or bffl for best friends for life but some use bgf for best girl friends etc etc.
If i was in your shoes i would go join them and you can still be friends or you can could have a talk with them both quietly and see where that gets you.
yeah its ok there are plenty of boys and girls that are best friends.
Tell your friends to grow up!
yes. if the boy doesn't like the girl then they'll still stay friends if they're true best friends.
Best friends are best friends because they share similarities. Lazy or hopeless guys love their best friends' girlfriend because he would like the same kind of girl.
Certainly yes.
Their really isn't a "line" for it
You tell her and you talk