I feel like either Toritoribeさん or I have explained this point before (perhaps I'm wrong), but the key distinction with giving and receiving verbs is in-group vs. out-group. With くれる, the direction of the giving is from out-group to in-group. The most common form that this takes is some other party to "me" or "us", but that is not necessarily the only form this takes—this is the subtle nuance that Toritoribeさん is trying to explain here.
The sentence 親御さんは必ずあなたを助けてくれます could be in a book meant to be read by children or spoken by a counselor to a child. In this case, the author/speaker is putting themselves in the perspective of the reader/listener (I don't believe this is such a foreign concept; it can happen in English as well, though in this case it won't manifest itself in the language since English giving/receiving verbs have no directionality). Therefore, the recipient of the benefit is not the speaker (the author/teacher) but rather あなた (the child being spoken to).
The same with 「彼女は彼が来てくれたことを喜んだ」. Imagine this sentence being written in a novel or something. It is written from the perspective of 彼女, so it would be nonsensical to interpret it as "She rejoiced that he did me (the author?) the favor of coming."
So yes, absent context and in most cases, it is fine to interpret the recipient of the benefit as "me" or "us", but just understand that there can be exceptions where the "in-group" can shift depending on the perspective of the sentence.