We've known what happened to him (or at least, that Bedalia murdered him herself) since towards the end of the second season following heavy foreshadowing in the first season, and we knew in the first season that Hannibal and her covered up the patient's murder as she comments to Hannibal that Crawford came asking about the incident regarding the patient and that she lied in order to protect them both. This season, we've seen precisely what happened in relation to the patient, both in regards to how Bedalia killed him, and how Hannibal assisted her in covering it up.
The patient was an individual who approached Lecter with mild depression, and with Lecter's meddling ended up deeply depressed and unable to sleep. He, simultaneously, was highly suspicious of Lecter, and was told to seek DuMaurier, Lecter already being very close to her and knowing that she, too, in many ways, is like him, and has a deep fascination to him.
During the course of his care, Lecter administered 'photo-therapy'. We already know that Lecter did this to Will, too, to administer seizures, and experience time losses, while we know he did it to Miriam Lass as part of her brainwashing. We know, from the patient's comments, that during the 'photo-therapy', the patient began to choke on his own tongue, and Lecter sat indifferent, presumably to see how it would play out. Ultimately, he came to, and he survived. We can assume that as a result of this experience, he ultimately ended up ending his therapy with Lecter, and was then referred to DuMaurier. The patient also knew that DuMaurier was covering for Lecter, or did not have the patient's best interest at heart, as she wouldn't take action based upon what he said despite DuMaurier saying that she should. What ultimately ends up extremely important is the role the light plays. When he gets angry, and confrontational, and begins calling her out on what he's done, he begins to choke. Presumably, whatever Lecter was doing with the light therapy, was to make him susceptible to beginning the response to choke. Perhaps he had some underlying condition that made him more likely to do so, perhaps Lecter did something during the loss of time to encourage the response to choke and it just so happened that he began choking when in therapy with DuMaurier.
The patient could have survived. Not only is there the possibility that he would have recovered alone without interference as he did without Lecter's previous intervention, and in addition to this Bedalia was, in fact, trying to save him at the beginning. She exclaims this numerous times as he resists, and she put her hand into his mouth initially to try and save him. At the moment when she, presumably, catches his tongue, and has power over his life there's a pause, and it becomes clear that she has accepted her initial urge to crush him, something that previously she had always managed to move beyond, and shoves his tongue into his throat, killing him. Did she do it to protect Lecter? Possibly, but there is also the implication that she did it just because she wanted to. This scene then continues with Lecter's appearance as we've seen in a previous episode (the first, I believe).
So, they key to it is what Lecter did during the light therapy. We already have evidence that he does this to induce blackouts and aid in hypnosis, so presumably he did something (whether surgical, or with influence) to increase the likelihood of this happening, and then when the patient became irate, he triggered the response which DuMaurier used to kill him (and DuMaurier then realising that Lecter purposely put her in the situation, and accepting his help to cover it up).
It's not really that much of a 'grand revelation' really. It adds a great deal of insight into DuMaurier's actions, and clarifies what happened (whether or not the patient being manipulated into choking on his own tongue is to one's liking is another matter entirely), while simultaneously (once again) reinforcing Will's position on murder by providing a contrast with Bedalia (one suspects it provides a contrast with Hannibal too, but there is also the argument to be made that 'helping' the bird, and 'crushing' the bird, could be considered one and the same; in the context of it I believe it''s clear that we are meant to imply that Hannibal's reaction would be to crush the bird, emphasised partly by the Ottoman last season and the context of the discussion).