Sounds utterly bizarre. But I'd probably break it down into two spheres:
1 - They don't respect our friendship enough to mind the boundaries that come with it, and they don't deserve to hold title of Best Friend.
2 - They have momentarily lost their trolley.
If it's a question of advice, just talk to them about it. They could be a little mixed up or confused at the moment, which happens to the best of us, or are an inconsiderate jerk who isn't worthy of being your friend. Flirting with somebody's family or exes is not a hallmark of a good buddy. Still, everyone deserves a chance to explain themselves.