Often it is to me, yeah. Obviously you can judge it on a case by case basis, because sometimes false accusations do happen. But in the grand scheme of things, sexual assault happens so much more frequently than you seem to believe. I couldn't imagine one of my friends confiding in me that they were assaulted and my response being "well show me your evidence to prove it before I believe you". I wouldn't give anybody I know that response, and I don't think I should treat the woman in this case any differently until there's a reason to.
And the fact of the matter is, it does actually require evidence to prove it in court. If it didn't, the justice system would be flooded by similar cases, but people for reasons such as that (and others) don't bring it past maybe their friends or social sphere.