English Translation
I forgave his mistake.
Incorrect translation.
過失 means “a mistake cause by a negligence of duty of care”. It’s primarily a legal term with the specific meaning. 過失 excludes intentional mistakes.
We usually don’t use 過失 in this context. The legal enforcer is the one who can dismiss one’s 過失. “I” as an individual (mostly the victim) cannot forgive 過失 unless “I” am the judge.
If the sentence is in the context of personal conflict, replace 過失（かしつ）with 過ち（あやまち）.