That’s usually a sticky situation that involves a lot of different factors. For instance, if the friend and the ex only dated for a month and they broke up and they’ve both moved on, then I wouldn’t really think anything of it. But if they dated for an extended amount of time, like a couple years or whatever, and they recently broke up then I would say to take a step back. Like if one of my best friends wanted to date an ex, I wouldn’t care. There would be like 3% of me that would think it’s weird, but I’ve clearly moved on and I’m in a new relationship of my own. But these have been my ex’s for 2+ years. It also depends on factors like, was it mutual? Was there closure? How heartbroken was your friend? Are they over their ex? Have they moved on? Those two broke up for a reason, and it’s probably because there is someone better out there for each of them. But sometimes giving it time is the smartest thing, because if it’s meant to happen there is no point rushing into it and stepping on best friend’s toes/feelings.