Receiver of a もらう verb

Question

In a movie I saw this sentence:

私の祖母も彼女に手紙を書いてもらったことがあるんです

which was translated in the subs as:

She had also written letters which my grandmother received

As context, there are four characters: the writer, the speaker, the speaker's grandmother and the speaker's grand-grandmother. What happened is, the grand-grandmother hired the writer to write letters to be sent to her daughter (the speaker's grandmother), and the speaker is talking about this. So:

• Grand-grandmother: hires the writer.
• Grandmother: receives the letters

Given the context, the translation in the subs sounds right, but as far as I understand 私の祖母も彼女に手紙を書いてもらった would mean that the grandmother asked the writer to write those letters, since marked by に is the doer of the verb, and by も the one who asked to do the action.

Is there anything in the sentence indicating that's not the case? It's just the context indicating that's not the grandmother that asked to write, but rather she received the letters? I'm not sure about the grammar of this sentence, it seems at odd with what I know about a てもらう sentence (which, admittedly, still causes me some confusion).

Answer 1

Your understanding is correct. Unless you already know what actually happened, it is practically impossible to understand from that sentence alone that the grandmother didn’t herself ask the writer to write the letters.

I guess the use of もらう is still acceptable here because, as the recipient of the letters, the grandmother could be seen as the ultimate beneficiary of the act by the writer.