True, if everything ended well it might be easier and your close friend might even encourage you to pursue this relationship.
Yet I couldn't help but thinking that my friend imagines that I already desired his ex while they had been together. Even a long and deep friendship can be harmed despite the sincere desire of everybody to deal with such a triangle issue maturely. Good intentions don't suffice.
To be quite blunt, you can meet so many people so why pick one of the few ones who have been ex-girlfriends of a good friend of yours and risk damaging your friendship?
On a side note, I frankly admit that there is something slightly chauvinistic about this 'ex of a friend is taboo' as it presupposes a notion of "exchanging women" and the exception to this in the case of your close friend which kinda elevates the male friendship above "petty sexual stuff".