So by best friend HAD a boyfriend and he dumped her and now he asked my out! And I told her and it's a huge problem and I don't know if I should say yes or no to him!

Even if she gives you her blessing, this could open a can of worms you don't want to open and it could harm your relationship with your best friend. (Who, ultimately, is more important than a guy) --there are other fish in the sea who haven't dumped your dear friend.

I told him yes but me and my best friend are cool:)