A person being enlightened by a non-enlightened

The following comes from the Khemaka Sutta (SN22.89).

Ven. Khemaka had been ill. When he was asked, he said (I paraphrase), "I am feeling pain and I am not getting better."

For this, the elder monks asked if Ven. Khemaka takes it that the five aggregates are the self, to which Ven. Khemaka replied that he doesn't assume that the five aggregates are the self.

For this, the elder monks said Ven. Khemaka must be an arahant but then wonder why he said "I am ill".

Ven. Khemaka replied that although he doesn't make the assumption that the five aggregates are the self, but still he hasn't overcome "I am" yet.

"Just like a cloth, dirty & stained: Its owners give it over to a washerman, who scrubs it with salt earth or lye or cow-dung and then rinses it in clear water. Now even though the cloth is clean & spotless, it still has a lingering residual scent of salt earth or lye or cow-dung. The washerman gives it to the owners, the owners put it away in a scent-infused wicker hamper, and its lingering residual scent of salt earth, lye, or cow-dung is fully obliterated.

"In the same way, friends, even though a noble disciple has abandoned the five lower fetters, he still has with regard to the five clinging-aggregates a lingering residual 'I am' conceit, an 'I am' desire, an 'I am' obsession. But at a later time he keeps focusing on the phenomena of arising & passing away with regard to the five clinging-aggregates: 'Such is form, such its origin, such its disappearance. Such is feeling... Such is perception... Such are fabrications... Such is consciousness, such its origin, such its disappearance.' As he keeps focusing on the arising & passing away of these five clinging-aggregates, the lingering residual 'I am' conceit, 'I am' desire, 'I am' obsession is fully obliterated."

So, Ven. Khemaka explained that although he doesn't hold the fetter of the wrong view of identity view, still he has a lingering sense of "I am" due to clinging and conceit. He is actually a stream winner (or higher) but not an arahant.

At the end of the discussion, they all became arahants including Ven. Khemaka himself:

Gratified, the elder monks delighted in his words. And while this explanation was being given, the minds of sixty-some monks, through no clinging, were fully released from fermentations — as was Ven. Khemaka's.

One story I remember is from Questions of King Milinda, where Nagasena himself not yet a stream-enterer, preached Abhidharma to a lay lady - as a result of which both the lady and Nagasena have achieved the stream-entry:

Now a certain woman, a distinguished follower of the faith, had for thirty years and more administered to the wants of the venerable Assagutta. And at the end of that rainy season she came one day to him, and asked whether there was any other brother staying with him. And when she was told that there was one, named Nagasena, she invited the Elder, and Nagasena, with him, to take their midday meal the next day at her house. And the Elder signified, by silence, his consent. The next forenoon the Elder robed himself, and taking his bowl in his hand, went down, accompanied by Nagasena as his attendant, to the dwelling-place of that disciple, and there they sat down on the seats prepared for them. And she gave to both of them food, hard and soft, as much as they required, waiting upon them with her own hands. When Assagutta had finished his meal, and the hand was withdrawn from the bowl, he said to Nagasena: 'Do thou, Nagasena, give the thanks to this distinguished lady.' And, so saying, he rose from his seat, and went away.

And the lady said to Nagasena: 'I am old, friend Nagasena. Let the thanksgiving be from the deeper things of the faith.'

And Nagasena, in pronouncing the thanksgiving discourse, dwelt on the profounder side of the Abhidhamma, not on matters of mere ordinary morality, but on those relating to Arahatship. And as the lady sat there listening, there arose in her heart the Insight into the Truth, clear and stainless, which perceives that "whatsoever has beginning, that has the inherent quality of passing away". And Nagasena also, when he had concluded that thanksgiving discourse, felt the force of the truths he himself had preached, and he too arrived at insight he too entered, as he sat there, upon the stream.

Then the venerable Assagutta, as he was sitting in his arbour, was aware that they both had attained to insight, and he exclaimed: 'Well done! well done, Nagasena! by one arrow shot you have hit two noble quarries!' And at the same time thousands of the gods shouted their approval.

An alternative translation of the same paragraph:

And venerable Nagasena pronounced for her a sermon on the Abhidharma, about things deep in their meaning, transcendent, dealing with emptiness, [in accordance with the law of dependent co-arising. That is to say: When this exists, that exists; when this arises, that arises.] And right there and then, even before the lay lady could raise from her seat, there arose in her the dust-free, crystal-clear Vision of Dharma: "Whatever is conditional can be controlled". And venerable Nagasena himself, as he was gratifying the lay lady with his sermon, reflected upon the Dharma he was expounding and had himself attained the insight; and even before he could raise from his seat, he had attained the fruit of stream-entry.