First of all, there’s no defending this man.
You’re married, yet you’re seducing your wife’s best friend in your wife's restroom?
Even if you intend to take a second wife, it should never be your wife’s best friend unless she genuinely approves.
Both parties are at fault, but the greater responsibility lies with the man, who is expected to exercise better judgment than the woman he’s involved with.
No woman deserves a man like this.
Stop defending his stupidity.