Hello @0verwhelmed and welcome. I'm sorry noone has yet acknowledged or answered your post.
As someone who's experienced psychosis, and who also looks out for a family member who's unwell, I appreciate your concern. You're a good friend to your housemate. I suggest you continue to stand by them. When appropriate gently encourage them in seeing their gp or mental health practitioner. Other than contacting the local crisis team or emergency services if they're in crisis it's hard to intervene. I trust they'll recover before reaching that point. When they are well again, I wonder if they'd be open to discussing a contingency plan of how they'd like things to be and what they'd like you to do if/when they are unwell, and how they might recognise when it would be wise to seek help.
It's not easy @0verwhelmed and I wish you well. Look after yourself, seek your own supports, reach out to others, please maintain hope that things will get better. There's a SANE Helpline staffed 10-10 M-F if talking to someone would help.
Dimity