It is scary when someone you know and love becomes 'different' but it's REALLY important to stay connected to roadmaps of normalcy, that person is not themselves and themself might be gone for a while. The person inside a familiar body IS talking nonsense and gibberish. Sometimes they like it, if you buy into the psychosis and pretend their reality is real, with them. I have found it to be an effective way of 'socialising' with a person in psychosis. Depending on the course of their situation it can take 3 months - 1 year to stabilise and, they are embarrassed too so in my experience it was most important to treat the person as 'normal', learn their new language of true distress vs attention seeking and try to keep their sense of humour and personality alive. It is dehumanising treatment and being seen as a problem which makes most peoples lives fall apart. Yes they are "a problem" but their situation is worse than yours, they need help to remember whats real in reality and also to remember themself, and its ok to be themself. Moving forward you start to learn when it comes over them and then you can escalate assistance or just let them be as long as they arent harming themselves or others.
I know a gentleman with significant voices and visions who has lived a harmless solitary life, all his life, and is a gifted painter. To talk to him you would never know he lives in a different world until you realise 100% of his world is in his head, with information coming in via books and out via paintings, writings and making objects. This is obviously different to someone in a relationship but, survival is the baseline of life and for a person in psychosis first they need to survive and after that they will worry about relationships, I hesitate to give direct advice BUT I would think, like a stroke you might not be able to get to the person you are in a relationship with, immediately, and will have to deal with first 'betterness for that person, stabilisation of that condition' and 'whats left of the old relationship'....
All the best, remember like Swimming lessons: in order to be able to save the other person, you have to have flotation for yourself.