I’ve had several friends who went on to suffer from various mental ailments and the dilemma was always whether you should tolerate their unacceptable actions because of the illness or shun them as you would with anyone else. One friend who is (I think) bipolar would say terrible things to his wife in public. Many of our friends have ostracised him, while I only tolerate his company out of loyalty to his wife, who is my friend. She has stood by him for about 20 years.
At work, one of my co-workers started behaving strangely and I remember asking her if she was on drugs (half-jokingly) and feeling bad when it turned out she was having a major psychotic episode. She went on to suffer from severe depression for a long period until she was given ECT as a last resort and it seems to have helped her.
The one who “went crazy on me” was another friend from work who had been severely depressed to the point where almost all hope was lost. As a last resort, like our former co-worker, she was also given ECT and it ended the depression, but that combined with the anti-depressants she was taking, pushed her into a state of euphoria and that’s when she came to stay with me. I had recently given birth and was not at my most assertive.
She had tried to visit several times and I kept putting her off, saying it was too soon since the baby was born, because she was a demanding, high-maintenance friend at the best of times, but part of me felt guilty and was worried that she might think that I was rejecting her because of the mental illness, so against my better judgement I set some parameters - maximum length of stay, earliest date I could accommodate her, the fact that she would be expected to do her own thing and that I did not anticipate being her tour guide… it was still a total disaster which ended up with her saying terrible things to me and declaring our friendship over (much to my relief) and like other posters here it did end a couple of years later with her apologising, but by then I had gone through too much to be able to go back to being her friend. The main reason being that it dragged me down emotionally at the time when I needed it least and must have affected the quality of my parenting when my son needed it most.
I used to get worked up about this for years after it happened, and now that about seven have gone past I can finally say I’m calmer about it, to the point that I could just about tolerate the thought of having a conversation with her again. 
