Speaking for myself here
None of my ex's are my Possession..
therefore flirt away as it isn't any of my business and if i get my little feelers hurt that's my shit to deal with
Im also gonna go out on a line here and stand up for the stranger in this scenario because no where in the original post did it denote that the 2 were "friends" or best friends for that matter simply that they had on occasion to bump into each other....
I wont live by a code where my ex is suppose to be a possession nor will i live by a code where your ex is your possession unless of course there is a contract with your ex and then respect to that contract simply applies...that's a whole 'nother topic though
in any case perhaps the OP's friend who was "crushed" need pull up her big girl panties and move on already.