And was surprised when it was revealed to him that he was therefore a rapist
He had every right to be surprised. On the basis of his account he was not a rapist. According to Evans, McDonald asked the victim if Evans could have sex with her. She answered that he could. She then asked Evans to give her oral sex, following which they briefly had vaginal sex. There is nothing in Evans account that makes him a rapist. The prosecution is based entirely on the victim's lack of recollection of the events, using that to argue that she was too drunk to consent and that Evans' account is false. His behaviour was not ok but the statements given by Evans and McDonald do NOT describe rape, nor can they in any way be described as confessions. They describe an encounter between consenting adults.
why did the prosecution not make more out of the friendship between the witness and the accused
There does not appear to have been much of a friendship for them to make anything out of. I presume we are talking about Owens. The wording in the appeal judgement states that he had been acquainted with Evans which does not suggest a close relationship.
And why do you think there is such a disconnect between the prosecution thinking they had a weak case and the general public (including us mumsnetters), etc who think it's a strong case
There is a lot of misinformation circulating on social media about the case, most of it beefing up the prosecution case. See, for example, the comments above from ageingrunner who thinks Evans' statement amounted to a confession. I've seen that one quite a lot but it simply isn't true. It shows him behaving very poorly but it does not show that he was a rapist. I've also seen other comments such as the victim consuming a large amount of drink from the mini-bar in the hotel room (she didn't), being bundled into a taxi by McDonald (she wasn't) and so on.
This case has been taken up as something of a cause by some anti-rape groups who see this as a chance to redefine what being too intoxicated to consent means. This, of course, means they don't want to believe Evans is innocent.
Then there is the fact that the original jury convicted Evans. There are always those who believe that every conviction is correct and that there are no miscarriages of justice. Whilst people seem more willing generally to believe that miscarriages do happen, there seems to be particularly strong resistance to believing that any rape conviction could be wrong, possibly due to the oft-repeated 6% conviction rate statistic.
Another big factor, I suspect, is the "I believe you" meme. A lot of people seem to think that the victim accused Evans of rape and/or said that she was too drunk to consent. In fact she has never said either of those things.
And, of course, we now have some newspapers questioning the verdict. It makes for good copy and helps to sell newspapers (as has this case throughout) but the one sided (and frequently wrong) presentation of this case in some parts of the press does not serve the interests of justice.