It was obvious what Lea had meant. He was saying that Arnold is serving Cole and it was Cole's instructions that made him get close to Sandy.
Arnold could not defend himself at all.
He originally had ulterior motives for getting close to Sandy but afterwards, he really fell in love with Sandy.
But both of them had been tied down by many conflict of interests. He did not know that Lea had plotted a scheme before he left and still chose to come to Sandy's side by following with the flow.
However, it caused many troubles to entangle him, causing him to be unable to break free easily.
But even though it was a decision beyond redemption, he still wanted to become closer to Sandy.
Yet, his sincerity now could not change the ulterior motives he had when he initially got close to her, and it could not change the fact that he was fulfilling the duty as a part of the Snow Fox Clan and taking orders from the Fox King.