Who does Fernand encounter after leaving Mercédès’s home, and how does this meeting affect Dantès’s future?

I'm thinking you mean chapter 3. Ferdinand meets Danglars and Caderousse. Fernand realizes that Danglars despises Dantes also, though he does not know the root of Danglars' hatred. Danglars orders more wine for Caderousse, who at this point has little sense left in him. He also asks the waiter for pen, paper, and ink. He writes a letter to the king's attorney incriminating Dantes as a Bonapartist agent. The letter shall be anonymous, and thus Fernand will not look guilty to Mercedes. Danglars dupes Caderousse into thinking the letter a jest. He throws the letter in the corner, yet he knows Fernand will fetch it and deliver it once he gets up to bring the drunken man home.