Compromised said:
If it is an emergency, 911.
If it is not an emergency, any outpatient clinic at a hospital.
For situations where they need help, and the person is not an immediate danger to themselves or others, you can also take them to an ER and ask to speak to the crisis team. They can help evaluate the situation, and get advice from an on-call psychiatrist. This can help get needed immediate help, drugs if appropriate. (Well, as immediate as anything in ER's these days.)
Been there, done that.
The hard part is getting follow-up care. Unless the hospital psychiatrist or clinic agrees to pick them up as a patient, they'll need to follow-up with their GP. The GP can work medication regimes, etc, and give a referral to a psychiatrist or clinic, but that takes time. And they have to find a doc or clinic with capacity.
CAMH has programs for out-patient care, as well as hospitals like Trillium in Mississauga. Websites have details.
Remember, there is only so much you can do for another person. Look out for yourself too.