It's not your fault pal, some are great at hiding it.
You just need to remember it's just like any other illness and mostly boils down to chemicals. Just like if your friends body didn't have enough white blood cells or too much sugar in the blood.
From faulty mood regulators to depleted serotonin stocks, dopamine running on fumes or receptors out of action, the brain and its components are just like any other part of the body. If a bits not working as it should be we are in trouble.
The task is now to get your friends components working correctly. Lifestyle, meds, therapy etc.
Looks like they've made the first step and that's the biggest step of all.
You just hang in there, they'll probably just want you to be you. They might not want their pal all of sudden changing and fannying round them... or they might.
Glad they are getting help and remember it's nobodies fault. Brains are fucked up.