Translation:The window is broken, but she had forgiven him quickly.
i couldn't say for sure whether the use of the tenses in the german here is unusual but in the English it is a bit. If it is normal to suppose that we are to relate the two clauses in the sentence given, the present tense would usually call for the past and not the past perfect, i.e. "...but she [quickly] forgave him [quickly]." (unless, of course, we are to imagine that a third invisible event/clause in the past tense is being referred to). Even this, out of context, sounds a little strange since the sequence of events is unusual. More obvious would be, "the window was/had been broken [by him], but she quickly forgave him." I expect in German it is the same.
I agree, I don't know why you'd say 'had forgiven him' here, rather than just 'forgave him'.