My mate constantly plays Master of Cruelties which can only attack alone,
Master of Cruelties isn't forced to attack, but if it does, it must do so alone. If you control another creature with an ability that says it must attack if able, that creature must attack and Master of Cruelties won't be able to.
If Master of Cruelties attacks alone, another creature entering the battlefield attacking will have no effect on it. Master of Cruelties continues to be an attacking creature. The other attacking creature will assign combat damage normally, regardless of whether Master of Cruelties's last ability triggers.'
He always makes the mistake and attacks with multiple creatures with Master of Cruelties. What would the ruling be on this? I am unsure but I would guess maybe it just taps out?