Translation:It is necessary that his friend is informed of it.
I must agree with smearedink. I'm all for mixing it up but only after one has had a chance to see the tense demonstrated correctly. Perhaps there should be two Subjunctive Past lessons (groan!): the first containing only good examples of legitimate Subjonctif passé sentences and the second with some diversity of Present Subjunctive and other "distractors".
I groan too, 4 years later, but I think it IS confusing to put them together. Seeing the past subjunctive, I was taught that it refers to a time previous to the main clause. If the main clause is present, then the past subjunctive should translate as "has"; if the main clause is past, the past subjunctive should translate as "had," to place it in time as previous to the main clause. But those passive constructions are confusing, if not also explicitly taught, since they look so much like the active constructions, but are really occurring simultaneous with the main clause.
Evelim, you are right. There is no "past subjunctive" in that French sentence. Another English equivalent, which Duolingo should add if they have not already, is: "It is necessary that his friend be informed of it" or even "His friend has to be informed."--Larry Porter
That looks right to me (except for the missing accent of course), but I am not an authority. However, since the auxiliary verb in the subjunctive is être, the past participle must agree with the subject, which is son amie here, a feminine noun. So informée would appear to be the correct participle to use.