Cane and Iris let Rei came over because they wanted to have as many options as possible to save Lou. They had their main decision, but they were open to another suggestion.
If this was not for the merchant, they would let the people from the Holy Kingdom stayed out of this, because they got the hang of the situation, where they were sure they could end this war as soon as possible, since they had covered their basic, such as food supply and the fighters in the battle.
Cane got Cezi on his side and his warriors thanked to Iris, not to mention his mate got the loyalty from the Blue Moon pack thanked to him.
And with the merchant on their side, they helped Cane to gather magic users, who would arrive within a month from now.
They could carry on without the involvement of the Holy Kingdom. But, Lou was in dire situation and as much as they didn't want to have anything to do with them, they couldn't do so.