Side Note: Let your friend have as much space as they need especially if their actions point to some reservations about your new bae. Life is never black and white and there is no hard and fast rule that says you can never date a friends ex.
Can old friends fall in love?
Yes, love can turn into love. It happens a lot of times, and many claim that the transition from friendship to love is quite smooth. So yes, that great friendship with your best guy friend can turn into a romantic relationship. Your relationship with your best friend can change into a romantic relationship easily.
Is it bad to date your best friends ex?
The unspoken rules of girl code imply that its basically never OK to date your friends ex, especially your best friends former boyfriend. Unless you value your relationship with a guy more than your friendship, respect that your friend may not be thrilled you want to start dating her ex.