Who does Fagin think Nancy wants to meet with on Sunday night?

Are you referring to the news that Noah gives Fagin about Nancy's meeting on the London Bridge?

I found it..... Fagin believed that Nancy had found another man and was ready to leave Bill.

Fagin walked towards his home, intent upon the thoughts that were working within his brain. He had conceived the idea—not from what had just passed though that had tended to confirm him, but slowly and by degrees—that Nancy, wearied of the housebreaker's brutality, had conceived an attachment for some new friend. Her altered manner, her repeated absences from home alone, her comparative indifference to the interests of the gang for which she had once been so zealous, and, added to these, her desperate impatience to leave home that night at a particular hour, all favoured the supposition, and rendered it, to him at least, almost matter of certainty.