Hmm. I can't see a circumstance where I'd ever refuse to let a friend in my house, bu nor would I end a friendship over it. I can think of lots of different things that could've happened. Usually I am not in favour of excusing rude people with all kinds of 'well, maybe she has a disorder' possibilities, but in this case...well, to me it is just as unlikely that the friend is simply a rude person who won't let a friend use the washroom than that it's something a little different.
I have pretty bad anxiety at times, and while it's not about this issue in particular, it actually would make sense to me the friend might have a bit of a freakout/panic moment and not be able to properly explain what's going on, if it's something like a rather gross mess in the bathroom. I do think she should try to explain better, later on, if possible.
But to me, there are so many different variables that in an otherwise good friendship with someone who isn't otherwise selfish or rude, I would let it go.