Sarah, basically, you need to ask your friend to tell you the true and full facts of this case if you want proper advice on here.
Pleading self-defence is not as straight forward as some may suggest and in such a case it is a specific matter of law and for the courts to decide. Do you know why your friend is pleading not guilty i.e. on what grounds is he saying he has not committed unlawful assault?. If, for example he is claiming he was attacked and defended himself, why was it necessary to 'disappear' for a month instead of reporting the matter to the police. I note your previous reply to this question in that he said he did not know what to do but it just doesnt sound convincing.
Surely your partner has told you how he came in possession of the knife, what has he told you?
His bail was refused even though he surrendered himself to the police and yet the police and courts agreed to remand him in custody for trial. There are only a few reasons why bail can be refused, does your friend have previous convictions, is he of no-fixed abode. What did he tell you the reason was for being refused bail?
Also, you state the injured person has a previous history of violence using a knife, how do you know this?.
As already mentioned, this is a very serious offence and it is further aggravated by the use of a knife in public.