I know as someone who was a caretaker[0] for a partner, it's incredibly hard to separate making responsible choices for someone who can't always make them from your regular actual friendship or relationship with them.

If you were already friends with the person, I really don't see them ever wanting to treat you formally. In the worst case they might have to make the almost impossible decision between involuntarily committing you (if they think you are a danger to yourself/others) and losing a valued friendship, or not committing you against their better instincts to preserve their friendship and risking you harming yourself/others. (Not that you are necessarily in that category ever, but if you're making a decision like "Can I ethically provide treatment for this person?" you really have to think about worst cases.)

[0] I don't have any medical/therapy training, but I wound up spending a few months taking care of someone I was dating who was having a pretty severe manic episode. Very irresponsible of her doctor, in retrospect, who later said the reason he hadn't hospitalized her was because she had me to help take care of her/watch her--and we were both college students and he had met me once, briefly.