But HOW did he ruin Mercedes? He already told her (in "bread and salt") that he's forgiven her for marrying another.

Fernand had plenty of blood on his hands... betraying Ali Pasha, a ruler that he served, resulted in the deaths of tons of Janina defenders. He took bribe money from the Turks and helped himself to "slaves" (like Haydee and her mother) and sold them to line his own pockets. That, plus ruining young Edmond's life with 14 years at Chateau D'if in order to get Mercedes for himself. Of course he deserved his fate.

Dantes (as the Count) wanted to help Mercedes. He offered plenty of financial assistance, but she refused it, only accepting the 3000 franc dowry and a roof (old Dantes' place). As the widow of Fernand, she voluntarily donated the Morcerf property and money to the poor and walked away from it all. Her own decision and choice. Not the Count's doing. If she wanted to, she could still be living in that nice house in Paris and not embarking on that pitiful journey to get to Marseilles to start over (from the bottom).

As we had left her, she would only accept the Count's help "if Albert approved it". So she was an indirect victim of the Count's revenge, but she had plenty of escape hatches and resources that she refused to take. Like the saying goes, "can't help those who won't help themselves."

IRL, people deal with this all the time. Family members die. Tragedies happen that might make you temporarily homeless (fire, flood, earthquake, disaster). You can deal with it, and tap every resource available (friends, relatives, existing bank accounts, insurance payout) and rebuild your life, or you can sink into a deep, dark depression and will yourself into an early grave. I am sad to say that for Mercedes, the latter is quite probable.