None of the quoted interpretations has quite got the meaning of this passage.
The context of this quote is that Jane has just been humiliated by Mr Brocklehurst in front of the whole school for accidentally dropping and breaking her slate. She believes that everyone despises her:
“Helen, why do you stay with a girl whom everybody believes to be a liar?” [Chapter VIII]
But Helen points out that this is not the case: in fact, the majority of the school sympathizes with Jane, but they dare not express this sympathy because Mr Brocklehurst would punish them for it, if he found out:
“Mr. Brocklehurst is not a god; nor is he even a great and admired man: he is little liked here; he never took steps to make himself liked. Had he treated you as an especial favourite, you would have found enemies, declared or covert, all around you; as it is, the greater number would offer you sympathy if they dared. Teachers and pupils may look coldly on you for a day or two, but friendly feelings are concealed in their hearts; and if you persevere in doing well, these feelings will ere long appear so much the more evidently for their temporary suppression.” [Chapter VIII]
So in this context Helen means the word “friend” in this sense:
A person who wishes another, a cause, etc., well; a sympathizer, helper, patron, or supporter.
Friend, n. sense A.4.a. Oxford English Dictionary.
What Helen means is that so long as Jane does what is right (that is, what her conscience approves) she will not lack for people who sympathize and support her, even if it appears that “all the world” hates her. (We need to insert “appears” here for it to make sense, but this is clear from the context.)
That is the plain meaning of Helen’s speech, but she also means “friend” in its more usual sense:
A person with whom one has developed a close and informal relationship of mutual trust and intimacy
Friend, n. sense A.1.a. Oxford English Dictionary.
Her gesture of affection (“she chafed my fingers gently to warm them”) and kind words show that Helen also means that she believes that Jane has done right, and will always be Jane’s friend.
Helen’s use here of “conscience” is a foreshadowing of events later in the novel. Jane will face two difficult decisions, and in each case it is her conscience of what is right that guides her.
First, when she discovers that Edward Rochester is already married, and she has to decide whether to live with him as his mistress, or leave him for ever:
I asked, “What am I to do?”
But the answer my mind gave—“Leave Thornfield at once”—was so prompt, so dread, that I stopped my ears: I said, I could not bear such words now. “That I am not Edward Rochester’s bride, is the least part of my woe,” I alleged: “that I have wakened out of most glorious dreams, and found them all void and vain, is a horror I could bear and master; but that I must leave him decidedly, instantly, entirely, is intolerable. I cannot do it.”
But, then, a voice within me averred that I could do it; and foretold that I should do it. I wrestled with my own resolution: I wanted to be weak that I might avoid the awful passage of further suffering I saw laid out for me; and conscience, turned tyrant, held passion by the throat, told her, tauntingly, she had yet but dipped her dainty foot in the slough, and swore that with that arm of iron, he would thrust her down to unsounded depths of agony. [Chapter XXV, my emphasis]
Second, when St John Rivers asks her to marry him, and she has to decide whether to accompany him to India as his wife, or be parted from him under the shadow of his disapproval:
“Consent, then, to his demand is possible: but for one item—one dreadful item. It is—that he asks me to be his wife, and has no more of a husband’s heart for me than that frowning giant of a rock, down which the stream is foaming in yonder gorge. He prizes me as a soldier would a good weapon; and that is all. Unmarried to him, this would never grieve me; but can I let him complete his calculations—coolly put into practice his plans—go through the wedding ceremony? Can I receive from him the bridal ring, endure all the forms of love (which I doubt not he would scrupulously observe) and know that the spirit was quite absent? Can I bear the consciousness that every endearment he bestows is a sacrifice made on principle? No: such a martyrdom would be monstrous. I will never undergo it: As his sister, I might accompany him—not as his wife: I will tell him so.” [Chapter XXXIV]
In both cases Jane suffers initially (her homeless flight across the moors from Thornfield in the first case; the coldness and scorn of St John in the second) but finds that she does not lack for friends (her Rivers cousins in the first case; the widowed Rochester in the second), just as Helen had prophesied.