you're no longer with the ex, why would you want to get involved with their best friend? That's disrespectful, and it could cause a rift in their friendship. I would be upset and weirded out if one of my best friends ended up with my ex-husband. (not that I have an exhusband) but it's alot different than ex-boyfriend, especially depending on the seriousness and length of the relationship. It's definitely something that should be thought out.