when buttercup thinks westley is dead, she agrees to marry Humperdinck. since buttercup never would have dreamed of wedding another while westley was still alive, isn't it horribly unfair of him to call her ''unfaithful''

This is an opinion question. Some readers are going to think that Buttercup was unfaithful, while others will agree that calling her unfaithful is indeed unfair. Looking at chapter 4, I would agree that calling Buttercup unfaithful isn't fair. To Buttercup's credit, she doesn't want to marry the Prince. She's simply not in love with him, and he essentially forces her to marry him. He's the Prince, she's a commoner of the area, and she's required to obey him. Buttercup actually refuses his command a couple of times, and she even accepts that her end fate is death for refusing the Prince.

"I am your Prince and you will marry me," Humperdinck said.
Buttercup whispered, "I am your servant and I refuse."
"I am your Prince and you cannot refuse."
"I am your loyal servant and I just did."
"Refusal means death."
"Kill me then."
"I am your Prince and I'm not that bad—how could you rather be dead than married to me?"
"Because," Buttercup said, "marriage involves love, and that is not a pastime at which I excel. I tried once, and it went badly, and I am sworn never to love another."

The Prince then does something incredibly honest. He tells Buttercup that he isn't marrying her for love, and he doesn't need her to love him; he needs somebody to provide him an heir. So he gives Buttercup a choice. She can be rich, powerful, and alive as Queen, or she can be dead. Faced with that choice, Buttercup chooses to marry Humperdinck.

"So you can either marry me and be the richest and most powerful woman in a thousand miles and give turkeys away at Christmas and provide me a son, or you can die in terrible pain in the very near future. Make up your own mind."
"I'll never love you."
"I wouldn't want it if I had it."
"Then by all means let us marry."

I don't believe that she is being emotionally unfaithful to Westley or his memory. She doesn't hide the fact that she loved another and won't love anybody else.