I think dating your best friend's ex is never okay. Ditto a sibling's ex, or any other relative's ex, or the ex of
anyone at all who's been a long-term presence in your life.
Reflection Line wrote:But every once in a while, the universe speaks to a person and lets him know that, although it seems wrong at first, there might be a bigger reason your friend dated this person in the first place – maybe it was to connect the two of you, instead.
The universe speaks, huh? Are you sure it's not your gonads talking?
Any notion that 'the universe' wants you with a particular person, that it's written in the stars, that you were 'meant' (by whom?) to be together or any other new-age claptrap get-out clause shows an appalling lack of imagination. That kind of lunatic thinking has been used to justify all kinds of atrocities.
So you want to date your best friend's ex. At least be honest about it, and don't fool yourself it really can't be helped because it's your destiny.