I would urge extreme caution. I have been hospitalised for m/h reasons several times (back in the 90s). My issues have never been of a psychotic nature, in other words I never lost touch with reality. The people I encountered who had been ‘Sectioned’ were either going through an extreme psychotic episode or a danger to themselves (risk of fatally self harming).
These patients are in a state of serious vulnerability, and very often do not think they should have been detained in this way. This can sometimes mean that they are very keen to talk about the unfairness of their detention. And, whilst I am sure that it can happen that patients have been sectioned without sufficient reason, it is not usual. I don’t know where this person stands in this regard, but the crisis must have had some severity for detention to be on the table.
For both of you, embarking on a romantic attachment would be a very bad idea. In your case, it is very possible (even probable) that their well-being of a precarious nature, requiring the right conditions to be present (meds, lifestyle, support). You need to think very carefully about your ability to support this; being involved with someone with these requirements has the potential to be emotionally draining, with situations that feel impossible to navigate. You may have that capacity. However, it is something you need to be 100% sure of in order to safeguard your own mental health.
In their case, romantic attachments are potentially very harmful indeed. Living with a mental illness can heighten the seeking of a partner for a variety of reasons, up to and including as a day to day support. That’s not to dismiss them, but adding the additional dimension of potentially rocky romantic scenarios could negatively impact their stability.
All this is to say please be very fully informed of such a situation. My personal opinion would be: don’t do it!! For both your sakes.