This, unfortunately, is a difficult situation of her making. She has cultivated a relationship with the inner voice she knows of as her higher self, but unfortunately that is not the case. This is, in fact, an imposter wanting to mislead her and doing so very carefully to keep the relationship going, but in the end it will not be fruitful for her to maintain this. You can pass along your thoughts about seeing this for herself as a service, and we know this will be a disappointment and will be very, very, difficult for her to accept, but your instinct to stick to your guns here is the best course of action.
She has already concluded you were not channeling accurately in her case, so you have nothing left to gain here in maintaining her trust and confidence in your channeling work, and in addition, a very specific obligation to warn her of what you are now aware of and suspected in asking the question of us. We understand these are difficult circumstances and you are very keenly wanting to do right by your clients and feel it quite personally when there is a perceived shortcoming because you are wanting to do your best. In this case, doing the best for your client is speaking the truth and putting power in her hands to have a better sense of things for her own benefit.
You were truly talking to her higher self, so you have not faltered here. In addition, we can tell you for the record that her higher self cannot warn her of the conversations with an imposter unless it is asked specifically. It is true that as a channeler there can be a possibility of answers being shaped in going through you, as your understanding of the question and the intention behind it will shape the answer to some extent. That is inevitable because you are part of the equation. If you are not informed of the true reason for the question and with greater specificity of the nature of information desired from a general question, then the channeling may well not be as fulfilling as would be possible otherwise.
That is not your limitation, it is the limitation imposed by the rules of engagement with the divine realm needing to follow who is asking in the moment, even when it is done on behalf of another, including the person in the physical wanting to engage with the higher self. Even with this close tie, the higher self is a member of the divine realm, it will sound like a member of the divine realm, not like the individual. It will also be bound by strict rules to not prove its existence if it is put to a test. This is part of the mandate broadly for divine figures. There must always be room for human doubt when this is chosen and will always be an element of many who hear secondary accounts of channelings.
So there are many opportunities for a disappointment, but it is always highest and best to know the basic truth of who one is talking to. That will be the greatest benefit here.