First of all I would be shocked. I'm Bipolar and I know all about the ups and downs of a relationship when your partner finds out about a serious mental illness.
In my case, the knowledge broke my complete faith in a fullfilling future. My partner had to support me even though he didn't know how to. I was in and out of mental hospitals, he never came to visit me and we split up after 8 years.
My point....you never know what will happen after the diagnose. I turned out to have a severe form of Bipolar Disorder. My partner turned out to be someone who wasn't able to handle the situation. I blamed him for leaving me but I know now, he wasn't strong enough. He couldn't help it. We tried for five years to make the best of it.
Now that I'm stable for more than ten years, I can be in a healthy relationship with my loving new partner. It's never easy but at least I know all about keeping myself on track.
I would say to all the partners from anyone diagnosed with a mentall illness...don't blame yourself for leaving someone behind if that's all you can think of after trying life out with your sick loved one.
Most patients need to travel a long long road to stability before they're even capable of handling any relationship in general. Of course every situation is different but the person diagnosed with a serious mental illness, is responsible for doing anything possible, to get proper treatment and living a healthy life, instead of leaning too much on their partner.