The term "friendly fire" is weird, because it's not friendly. People being attacked by weapons wielded by their own side still die. Self-inflicted harm is pretty much the worst thing possible in combat, because it points to incompetence, disarray, or internal conflict. The question is what reasons there are for the mine field which was placed by the Russian army *behind* the Wagner troops, discovered as they were about to leave Bachmut.
It's hard to imagine that these mines were meant to kill Wagner troops. There is no scenario where that would not blow up in Putin's face, figuratively speaking. The explanation that remains is incompetence and panic to the point where one group of Russian combatants has no idea what another group is doing, and where nobody thinks enough to communicate. The fact that Prigosjin accuses Kremlin of malice is telling, however, and it's surprising that he is still allowed to roam free and criticize Putin openly. The internal conflict between Prigosjin and Putin is obvious, and right now it reflects very badly on Putin.
