Thanks all again for your input, plenty to think about.
As much as I am the first to admit my DP is not perfect, and has like everyone his flaws, I do understand why he is still visiting her. Again, this is maybe because of my mental health understanding, and have seen myself how volatile she is, and why he is worried about her. That said, I have thought for quite some time he isn't helping her - or himself - by visiting, though he had been reducing all his contact with her gradually.
I do understand what you are saying, and to some level get how it must look, but because I know both of them, it does make sense to me why is he doing what he is doing.
Again, when I was asked by her about who he was seeing, I didn't tell her it was me because I really believe she is mentally unstable enough do herself some damage, or at least upset him enough into thinking that.
And really... I don't think it matters if she knows. Part of me does get frustrated with it at times, but everyone else knows, so I don't feel like a big dark secret. However, most of me feels that she would kick up such a fuss if she did find out, that actually it would give her more power over him, and more impact on our relationship than she already has. I am not hidden away or anything; we go out together, we go on holiday, he is openly affectionate with me in public. Its just one person.
I do firmly believe that he is trying to protect her - but also feel that he is possibly not achieving that. Yet understand what he is frightened of. She is a grown woman, and should be trusted enough with the truth, but her reaction to a lot of things is not logical. I can see both sides (as can he, we have discussed it) and understand why he has drawn this conclusion. Letting her speculate he is seeing various women, confirming to her he is dating... Its then a big leap to naming one person and saying he's in a relationship.
I can't tell say what the big lies she's told in the past are as they might identify us (and if she is actually ill and hasn't told certain people yet, I wouldn't want to destroy her privacy). But they were big, harsh, and she got caught out. Some I witnessed myself, some I was warned about by friends, and others I heard of via the management team. Knowing what she had said and done whilst they were together, and afterwards, would hurt him, and I don't want to be the one to do that. If people who have known him longer than I have have decided its better he doesn't know, then I trust that judgement.