'If you love a person, you can forgive anything'
If you notice, when the husband says that line to Leslie was the first time you see her real emotions, she looks like she came alive out of a deep robotic funk.
"If you love a person, you can forgive anything"
She became relieved and came alive, I think, because in her obsessed lovelorn mind she suddenly realized she could forgive her lover, she was actually the whole time still angry at her lover for deceiving her and "cheating" on her. When he said that, she suddenly forgave her dead lover and was relieved she could forgive him since she loved him. She then wanted to die, since she realized she could have forgiven her lover rather than killing him. Alternatively, after she forgave her dead lover in her mind, she went outside to talk to the Asian woman who she knew was there waiting for her, to tell her she was sorry for killing him, that she had "forgiven" him, and was desperately sorry. She must have thought that the apology was what the Asian woman really wanted long ago when she sold the letter.