My attention was drawn to this post as a friend of mine, who developed anorexia before she hit puberty (and therefore has the body of a 12 year old), has recently started sleeping with much older men who she has met on a night out. It is obvious when looking at her that she is not healthy, and so I can't help thinking that the men she is sleeping with are taking advantage of her.
In my opinion, when somebody's mental health gets so poor that they are no longer considered competent by the health service they are under, then they cannot give consent.
This does not mean, however, that we should try and further disempower people with mental illnesses.
With the example of anorexia, the seemingly reckless behaviour of regular casual sex with seemingly unsuitable partners may be an attempt to regain control over a part of their lives, once the control of what they eat is taken away from them.
From reading the above thread, it seems apparent that depression still isn't recognised as a serious disease (I assume this is from the casual use of the word "depressed" which we use as an adjective to describe how we feel on a day to day basis.)
As a society, I think we must work on removing the stigma surrounding mental illness, and begin to recognise the severity of it as equal to physical illnesses.
By doing this, it will enable us to speak more freely about these diseases, and the issues (eg consent) that surround them.