Nelly goes to visit Wuthering Heights. Edgar, however, refuses Nelly's request to send with her a token of forgiveness to Isabella.
Edgar won't forgive Isabella because he holds her, as a woman, to a different standard
At Wuthering Heights, Nelly barely gets to see Isabella at all. Instead, Heathcliff asks after Catherine's condition and then asks Nelly to help him see her, adding that were he in Edgar's place he would never stop Catherine from seeing someone she wanted to see.
Heathcliff is telling the truth. His love for Catherine is so powerful, that neither jealousy, a need for propriety, or anything else would stop him from trying to give her what he thought she wanted.
Nelly refuses to help Heathcliff, who threatens to hold Nelly prisoner at Wuthering Heights and go to the Grange alone. Nelly gives in, and agrees to carry a letter to Catherine from Heathcliff.
Of course, Heathcliff is also willing to do anything to get what he wants. His actions are always powerful, whether noble or selfish.